The Comcraft CTR-144 FM transceiver - A "multi-mode" solid state 2 Meter transceiver from the early 1970s.

Comcraft
Logo

Back in 1972 an ad appeared in the Ham magazines for something that probably seemed amazing at the time:  A solid state transceiver that covered the entire 2 meter amateur band (144-148 MHz - plus adjacent MARS and CAP frequencies) and did not need crystals!  This was the Comcraft CTR-144.  This radio cost $389.95 according to an advertisement in the January, 1972 issue of 73 magazine and was later listed at $489.95 in an ad in a 1973 73 Magazine advertisement (see Figure 6):  These prices are equivalent to over $3000 in 2026 dollars - for a two meter transceiver!

About Comcraft

Figure 1:
The Comcraft CTR-144 - a VFO solid-state 2 meter
FM/AM transceiver from the early 1970s.  A ham today
would probably be at loss when trying to use it!
The large knob is for tuning the receiver while the "VFO
Tuning" is for the transmitter frequency.  Aside from the
"tune" and "load" controls the rest are self-explanatory.
Click on the image for a larger version.

I haven't been able to find much about the Comcraft Company, other than it was in Goleta, California and that it seemed to exist from at at least 1971 until some time after 1976 when they released one of the earliest synthesized 2 meter transceivers, the CST-50.  I suspect that they may have made products for other markets, but I haven't been able find additional information.

More about the CTR-144

The CTR-144 is a "multi-mode" 2 meter transceiver in the sense that it can operate using both FM and AM - the latter being still present on the band in the early 1970s, but on its way out of common usage.  Compared to radios made since, this is an "odd duck" in the sense that it has separate tuning for the receiver and transmitter, both using VFOs - although it can accommodate three internal and one front-panel crystal for transmit for frequently-used repeaters and those applications that required good frequency stability and accuracy (e.g. MARS, CAP).  In other words, it doesn't have a conventional switch for a +/- 600 kHz repeater offset like that of modern radios, but rather you have to tune the transmitter and receiver separately to the repeater's input and output frequencies, respectively.  Additionally, it has a built-in power supply which allows it to operate from 120 Volts AC as well as a 12 volt input and provisions for an internal battery pack for "portable" use.

Figure 2:
The power amplifier section of the CTR-144 - solid-state
from the early 70s.  The "tune" and "load" air-variable
capacitors are visible, as is the side-mounted antenna jack.
Click on the image for a larger version.

As with some other radios recently featured on this blog, I acquired this one from a silent key friend of mine - and even into the early 2020s, he'd occasionally use this transceiver on some of the local repeaters that didn't require a subaudible tone.  It also has a very wide range "S-meter" which, unlike meters on most modern FM transceivers, would give a repeatable, reliable indication of signal strength over a range of just a microvolt or so to extremely strong. ¹  Being able to operate from a 12 volt vehicle and combined with the S-meter's wide dynamic range also  made it a decent receiver for direction-finding not only with a small directional antenna, but to know when one was getting very close to the transmitter itself.

About the transmitter

The transmitter isn't particularly high-power:  The specifications state that it's rated for 12 watts input power with about 6 watts out. ² Considering the efficiency of early 1970s VHF power transistors, the output power of this transceiver is still in the range of 4-6 watts across the band, depending on frequency and supply voltage - plenty of power for local and moderate-distance repeaters when used with a modest outdoor antenna.

Figure 3:
Before the mid-late 70s there were no specific FCC rules
for the spectral purity of VHF transmitters, but current rules
still allow transmitters of that vintage to be used despite this.
Were it made today, the CTR-144 would not pass muster.
Click on the image for a larger version.

Out of curiosity I checked this transceiver's spectral purity on my service monitor (see Figure 3) and I wasn't terribly surprised at what I found:  While there were a few minor non-harmonic spurious emissions that were at least 40dB down from the carrier, the second harmonic was around -27 dBc - roughly 4 milliwatts.  While this would not be given a "pass" if the radio was made today (the current rules state that spurious/harmonic signals of a transmitter with 25 watts or less should be -40 dBc) there were no such rules prior to about 1977.  As noted in the current FCC rules, (§97.307(e)) a radio from this era is exempt from these rules but, if using a radio such as this causes interference, the onus is still on the amateur to resolve any issues.

If one looks at Figure 1 you'll see that there are two tuning knobs:  The larger one labeled "Tuning" - which is coupled to the slide rule that covers 144-148 MHz - is for the receiver, but the smaller one, labeled "VFO Tuning" is what sets the frequency of the transmitter and it is completely independent of the receive frequency.  What this means is that if you wish to use a repeater, the receiver is tuned to its output and the transmitter is set to its input frequency which, in the day, may have been a bit of a challenge:  The VFO Tuning (for transmit) knob doesn't have a great deal of tuning resolution on its dial so getting it "on frequency" back in the day probably meant setting the knob as close as possible visually and then getting "talked in" to the proper frequency.  When using this transceiver, my friend would have a frequency counter handy to allow him to dial in the transmit frequency precisely and monitor it for drift.

The transmit VFO itself operates from 7 to 9 MHz and this frequency is then mixed with a 65 MHz crystal to yield 72-74 MHz - and this is then doubled to the 144-148 MHz 2 Meter band.  With this scheme the transmit frequency could be reasonably stable as the most drift-prone component, the 6-9 MHz VFO, was operating at a low frequency.

A close inspection of  the front panel  in Figure 1 will reveal - along the right edge of the front panel - both a "Tune" and "Load" control.  For anyone using a radio with vacuum tube finals, controls labeled thusly would be familiar, but as this transceiver has a solid state amplifier these simply provide peaking/matching for final amplifier as indicated on the "S" meter rather than the "dip and load" of the older technology.

Figure 4:
The bottom of the CTR-144 showing the
rather hefty power transformer and, on top,
the doubler stage for the transmit chain.
The modulation transformer can be seen to
the left of the large, blue capacitor.
Click on the image for a larger version.

To produce frequency modulation it is the 7-9 MHz VFO itself that is modulated, but if a crystal is used, phase ³ modulation is applied to its oscillator's output prior to being multiplied by 18 to the desired transmit frequency.  For AM, high-level modulation is accomplished by using an audio amplifier to modulate the +12 volt supply for both the final amplifier transistor and its driver via the windings of an autotransformer - a rather "old school" way of doing it, but one that eliminates the need for another, higher-voltage power supply to achieve the desired peak output power. ⁴

About the receiver

While schemes to up-convert a low-frequency VFO for transmitting had been used for a long time, the way this transceiver goes about receiving is a bit more interesting and complex.  While more modern analog receivers would generate a local oscillator near the receive frequency and convert the desired signal to one or more lower-frequency IF (Intermediate Frequency) stages where a single channel would get demodulated, this works more like the receive section of a transverter in that the entire 2 meter band is is down-converted to the range of 14-18 MHz using "high-side" injection (e.g. the local oscillator is above the receive frequency) at 130 MHz from the same 65 MHz crystal as is used for transmit.

The receive VFO - the one tuned by the large knob with the frequency indicated by the slide rule - tunes across the 14-18 MHz IF to down-convert the received signal down to a fixed IF of 2 MHz where it is filtered and demodulated by a CA3075 integrated circuit.  The receiver front end itself uses some MOSFETs for amplification and mixing - a technique still used on superheterodyne receivers made today - and its sensitivity is as good as any receiver made today.

Using the CTR-144 on the air today

Figure 5:
The front end and receive section of the CTR-144.
The three-section air-variable capacitor tunes the local
that down-converts from the 14-18 MHz first IF to the 2 MHz
second IF where an integrated circuit is used for audio
demodulation.  In the upper-left is the MOSFET front-end
and filtering that give this radio good sensitivity - even by
today's standards.
Click on the image for a larger version.

Using the CTR-144 today is admittedly awkward compared to a modern radio, even if that the lack of subaudible tone for transmit wasn't an issue.  The receiver's slide-rule dial will get you "pretty close" to frequency, but unless the signal is actually present, you won't be able to tune it in for certain.  For simplex use, once you have tuned the receiver, there is a "Spot" switch that will inject a weak signal into the receiver that you can use to tune the transmit VFO to the same frequency by listening for and zero-beating it:  In this way the transmit and receive frequencies will be very close to each other.

Repeater operation is another matter. ⁵  Back in the 1970s you might have spent a few dollars to get transmit crystals for your favorite repeaters which would circumvent the problem of not knowing one's exact transmit frequency with the VFO.  But what if you didn't have a transmit crystal for the frequency?  Here, the "Spot" button is less-useful since, unless you happen to tune in someone's signal as they are transmitting to the repeater on its input frequency, there would normally be no signal at all to which you could precisely set its frequency.  While my friend, in later years, had a frequency counter in his ham shack so that he could determine precisely the transmit frequency, such a luxury that would have been unthinkable when this radio was first introduced. ⁶

Figure 6:
A Comcraft advertisement from
the April, 1973 issue of
73 Magazine
Click on it for a larger version.

Final comments

• When I first placed this transceiver on my workbench, it worked perfectly (although I haven't tried the built-in AC power supply) aside from two of the #47 dial lamps having burnt out.
• When the sensitivity was checked, it was only slightly deaf, but just a couple of minutes of time with the service monitor and tweaking the RF and IF stages brought it into specs.  It's interesting that while the manual (see link, below) has some "adjustment" information, it doesn't have a fully-detailed alignment procedure for the transceiver, likely because, as is true today, few people would have access to the necessary equipment to accomplish the task.  As it was, the manual and schematic informed me as to how it worked I was able to come up with a procedure "on the fly" to bring the radio back up to snuff.
  
• When 2 Meter FM first appeared, the amateur community had not yet standardized on what we now call "Wide" FM (e.g. +/-5 kHz deviation).  Different channel spacings and modulation widths could be found in different parts of the country, including +/- 15 kHz deviation - particularly on UHF.  The manual for the CTR-144 specifies a deviation of +/-7.5 kHz - yet another "standard"!
• The specified receiver bandwidth is 22 kHz which is somewhat wider than the typical 15-17 kHz for a "wide" FM receiver intended for +/-5 kHz deviation.  With this bandwidth - plus the probability of the receiver's tuned VHF frequency not being centered and an adjacent-channel repeater (15 or 20 kHz away, depending on the spacing in your area) - its use is likely to result in received interference.

Comcraft CTR-144 manual

I was fortunate enough to find the manual for this transceiver in my friend's files which has been scanned and may be found at this link:  Comcraft CTR-144 Manual (35 MB, .PDF).  At the time that this blog was posted, this manual was not to be found anywhere else on the Internet.

* * * * *

Footnotes:

1. On most FM transceivers, the S-meter will start indicating on very weak (and possibly noisy) signals and hit the "peg" (e.g. full-scale) even on a signal that is only modestly strong.  What this means is that for direction-finding purposes, most modern FM transceivers will give a "pegged" S-meter indicating even if you are some distance from the transmitter.  The CTR-144 on the other hand reaches full scale only if you happened to be right next to the transmitter while at the same time it starts indicating on a signal that is almost too weak to copy meaning that it could be used both at a great distance or in the final search for an unknown transmitter. 
2. Until the mid-late 1970s, amateur radio power limits were specified as input power to the final amplifier stage.  This method of transmitter power was likely an artifact of the days before RF power meters were common:  It's trivial to calculate the input power to the final amplifier if one knows the current and voltage, but that isn't the case for trying to measure RF power.
3. "Phase" modulation doesn't change the frequency directly but rather it "wiggles" the phase of the signal in accordance to the applied audio whereas actual "Frequency" modulation does change the frequency of the signal - in this case, by electrically shifting the VFO frequency:  Mathematically, the two are related since, if you could continually change the phase, frequency modulation would be the result.  With "FM" as used in the amateur service, one need only be able to modulate down to 200 Hz or so for voice meaning that with phase modulation, you could "temporarily" change the phase enough to effect a temporary de-facto frequency shift.  As the amount of change from a single phase modulator stage is quite small (perhaps +/- 60 degrees) this is usually applied at a lower frequency - in this case, at the 8 MHz crystal frequency and since this is multiplied by a factor of eighteen, our original +/-60 degrees of phase change becomes (18 * 60) 1080 degrees which is enough to appear as an adequately-modulated "FM" signal.  Mathematically, if one boosts the "highs" of the audio at a rate of 6dB per octave (e.g. doubling the deviation with every doubling of frequency) a signal generated using "FM" is indistinguishable from one that was originally phase modulated.  In fact, it is "PM" (Phase Modulation) that we use on "FM" as amateurs:  The "highs" in the audio are boosted (e.g. "emphasized") on transmit as they are the first to get noisy with a weak signal, and on receive they are restored back to their original level (e.g. "de-emphasized") which effectively reduces the amount of noise that is heard and makes weak, noisy signals more "listenable".
   
4. "High-level" modulation typically refers to applying the audio to the output RF stages(s) of a transmitter by increasing/decreasing the supply voltage.  Traditionally, this was done with an audio amplifier driving modulation transformer:  On peaks with 100% modulation the voltage applied to the final amplifier is approximately doubled, yielding the  four times the peak envelope power - but it also allowed the voltage to go to near zero at the other extreme.  On this transceiver, an audio amplifier drives an auto transformer (e.g. one with a single, tapped winding).  With no modulation, the 12 volt power supply for the finals goes through it unchanged, but with audio, the transformer action allows the requisite 24-volt(ish) peaks to be applied to the amplifier stage - all without needing the complexity of a 24 volt power source.  This method was "old" even when this radio was made, but it is simple and effective.
5. When this transceiver was made it was fairly rare for an amateur repeater to use a subaudible tone as doing so would mean that an amateur using it would not only have to have the proper crystals (for most radios of the day) but also a means to produce the needed subaudible tone frequency,  Later, in some areas subaudible tones were introduced when a geographically-adjacent repeater was on the same frequency to prevent users in an overlap area from bringing up both repeaters.  In some cases in amateur radio service, subaudible tones are used owing to occasional interference issues:  This is often considered to be a "band-aid" solution to a problem that is often due to the repeater's receive signal path not being able to adequately reject off-frequency signals (e.g. overload) - but sometimes it might be needed on a "busy" site where other users' gear isn't as well-filtered as it should be.
6. Back in the early 1970s a frequency counter of any sort would be a somewhat rare and expensive piece of test equipment - and this would be particularly true for one that could operate at VHF!  By the late 1970s and early 1980s technology had progressed to the point where such devices were more affordable and available to the average amateur. 

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This page stolen from ka7oei.blogspot.com 

 

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An ultrasonic superheterodyne receive converter (e.g. "Bat Listener")

In the mid 90s I decided to throw together what I called a "Bat Listener" - a simple receiver used to convert ultrasonic sound down to the audible range.

Figure 1:
The exterior of the ultrasonic receiver, complete with fancy
labeling!
Click on the image for a larger version.

Two types of circuits:

There are two common ways to convert a higher (ultrasonic) signal to the audible range, whether this is done using analog or DSP (Digital Signal Processing) techniques.

Frequency division

There are several ways to do this, the simplest being the "divider" type which digitally converts ultrasonic frequencies to audible by integer division of the input to a lower frequency.

The problem with this simple approach is that it does not preserve the amplitude (loudness) of the original sound since it must take the input signal, amplify/convert it to a series of logic-level pulses - which loses any amplitude reference - and do a brute-force digital division.  Additionally, if there are multiple signals present, for the most part only the strongest one will be converted down.

Clearly, one cannot "tune" this type of circuit:  A signal at 40 kHz will always be divided down by a fixed integer amount,  Let's say that the circuit digitally divides by 32:  That 40 kHz signal will be at 1.25 kHz.

Additionally, the direct "A-B" frequency differences between ultrasonic signals is lost, instead being "(A-B)/N" where "N" is the number of divisions.  In other words, the relative frequency differences between signals is not preserved.

Heterodyne conversion

The other way to do this is to convert the frequency.  In this technique, two signals - the ultrasonic to be converted - and another generated by the device (the "local" oscillator) are mixed together.  The result is an arithmetic shift in frequency.

The biggest advantages of this method are the fact that that not only are the differences in frequency preserved (e.g. two tones 1 kHz apart at ultrasonic will appear as two tones 1 kHz apart at audio) but the relative amplitudes (loudnesses) of the received signals are preserved as well.

Frequency conversion:

I chose to build a heterodyning receiver to convert the input frequency to a lower one.  This can preserve the amplitude and frequency relationships  - plus it is fully tunable, allowing one to choose the frequency range to convert to audible sounds - and since it is a simple conversion, multiple signals present will also be preserved.

When it comes to frequency conversion, there are two ways:  The simplest - direct conversion - would involve mixing a variable oscillator with the incoming signal and filtering/amplifying the resulting audio.  This has the advantage of being the easiest, and it is the method described in this article:

     April, 2006 QST article, A Home-made Ultrasonic Power Line Arc Detector - link)

A similar device - the MFJ-5008 - was made by MFJ Enterprises, but as this company is no longer doing business, it is found only on the used market.

While I could have easily built something like this a decade before the above article was published, as I'm sometimes wont to do I decided to make it a bit more complicated, constructing a superheterodyne converter.

While a direct-conversion receive simply mixes an oscillator with the desired signal to cause a frequency conversion, a superheterodyne receiver operates like a conventional AM or FM radio:  The desired signal is first converted to an IF (Intermediate Frequency) - and this IF is then converted to audio.  The advantage of the superheterodyne scheme is that filtering may be applied at the IF to limit the receive bandwidth - and since the IF is fixed, its width remains constant over the tuning range, just like that in a conventional radio/receiver.

Circuit description

Figure 2:
Schematic diagram of the superheterodyne ultrasonic receiver.
See text for a circuit description.
Click on image for a larger version.

 

As noted above, this circuit is more complicated than it needs to be, so make of it what you will!

VCO:

The heart of the unit is U1, the VCO (Voltage Controlled Oscillator) which uses the venerable CD4046 PLL chip.  Often used for frequency synthesis, we are using (only) the oscillator portion, which provides a linearly-tuned and fairly stable frequency source, adjusted by the voltage applied via R101 (and scaling resistor R102).  The values were chosen to provide an approximate frequency range of 125 to 185 kHz (more on this later) to allow tuning of audio signals from (ostensibly) 0 to about 60 kHz.  The actual tuning range is closer to 115-190 kHz as a bit of extra margin for the frequency range.

The only critical component here is C101 which should be a frequency-stable capacitor.  I used a polystyrene capacitor, but an NP0 (a.k.a. C0G) or silver-mica could be used, instead.  When I reverse-engineered this device, I noted that the marked capacitance value was unreadable, but back-of-the-envelope calculates indicate that a value of "about 150pf" should be in the ballpark.

R103, connected to the "R1" pin of U1, sets the approximate center frequency range while R104, connected to the "R2" pin - sets the lowest frequency - which important, since we want to constrain the tuning to 125-185 kHz.  Additionally, the low end of the tuning range was further refined by R102 on the "ground" side of the tuning potentiometer, which sets the minimum voltage that may be applied to the "VCOIN" pin.

The VCO output, a square wave, is buffered by U2, a hex inverter, and several sections are used to provide both a VCO signal and its inverted version to drive the mixer.

While the 4000 series CMOS chips throughout this receiver will happily run from 3-15 volts, they are operated from a regulated 5 volt supply - mainly to improve frequency stability and to provide a nice, stable voltage for a few other low-level circuits and to provide isolation from the main battery supply which will vary a bit, particularly at higher receive volumes:  This variance, if it gets back into some earlier stages, could cause instability of the receiver in the form of "motorboating" (low-frequency oscillation or pulsing) or some other type of feedback.

BFO:

Another circuit is the BFO (Beat Frequency Oscillator) which is used to convert the IF signal back down to audio - both being processes that we'll discuss shortly.  This uses an inexpensive 500 kHz ceramic resonator to form an oscillator using one of the sections of U2C, the signal being buffered by U2B.  This signal is divided-by-two using U3A, one half of a 4013 dual flip-flop - and then divided by two again using U3B, yielding a stable 125 kHz signal.  As with the VCO, two phases of this signal (normal and inverse) are available, this time using the "Q" and "!Q" outputs of the 4013.

Input signal path:

J1, a disconnect-type 3.5mm stereo jack is wired so that an internally-mounted electret "capsule" microphone is connected by default.  This microphone element (M301) is of the "2 wire" type or electret microphone in which a bias voltage is applied to the same pin from which audio is drawn - this voltage being applied via R301 from the 5 volt regulated supply.  The specific make/model of this electret element is unknown as it was selected from a small collection to find the best performer at ultrasonic.

Using a MEMs microphone

Since this article was first written, I replaced the electret with a more modern MEMs microphone as described in Another article:  Improving my ultrasonic sniffer for finding power line arcing by using MEMs microphones - link.  These types of microphones are capable of detecting from low audio frequency (a few 10s of Hz) and much higher - often through 100 kHz.

The schematic in Figure 2 now shows how an ANALOG MEMs microphone (see the link above for more information) may be integrated into the unit.  As these microphones are not tolerant of 5 volts, I used some "normal" (not "high brightness") GaAsFET Green LEDs:  D401 limits the voltage to 1.8-2.0 volts on the microphone's power supply while D402 prevents "glitches" from damaging the MEMS device that might get through via coupling capacitor C402 when an external microphone is connected/disconnected.  When everything was done, there was a very dramatic improvement in sensitivity above 15 kHz and several switching supplies near the workbench - which I couldn't hear before - suddenly became very audible.

Microphone amp and filtering

The signal from the microphone is applied to U4A which is wired as a unity-gain buffer.  For this, an LM833 is used, an inexpensive, low-noise dual op amp:  An LM358 or many other types may be used here as well - just make sure that it is is fairly low noise:  I'd avoid the use of the LM1458 here as it is quite noisy by comparison!

Section U4B amplifies the signal voltage by 10 (20 dB of power gain) and this signal is applied via R305 to a simple L/C high-pass filter consisting of C303, C304, L301 and L302 the latter two components being inexpensive 18 milliHenry inductors.  Certainly, an R/C-based high-pass filter could have been constructed using U4B, but I chose not to do that for some reason.

Figure 3:
Inside the ultrasonic receiver, constructed on
prototype board and having been modified
several times over the years, prior to the
installation of the MEMS microphone.
Click on the image for a larger version.

In simulation, the C303/C304/L301/L302 filter has a -3dB roll-off of about  23 kHz, it's down by 10dB at about 19.5 kHz, by 20dB at about 16 kHz and by 40 dB at 9 kHz and with the values shown, it's flat to within 1 dB between about 24 and 100 kHz.

The output of the filter is amplified by U5B - and then even more by U5A (which has a bit of roll-off from C307) to yield a whole lot of gain.  It's very possible that I over-did the gain here, but unless the signal source is quite close, there is no clipping observed on the output of U5A.

Its worth noting that a mid-supply voltage is created using R309/R310 to provide a "virtual ground" for the op amps and to maintain stability, it is heavily filtered by C306 and C302, each located near the respective op amp shown on the diagram.

Mixer and band-pass filter:

It is this next section that may seem unfamiliar to some - the use of a CMOS analog switch as a signal mixer.  For this, a CD4066 is used which consists of four separate analog switches.  The filtered and amplified ultrasonic input signal from U5A via C308 is applied to pins 2 and 10 of U6A/U6D.  When the respective signals on the control pins "VCO_A" and "VCO_B" go high, the switches are activated, and because VCO_A and VCO_B are inverts of each other, each of these switches is closed in turn.  The result of this is that the inputted signal is chopped up at the rate of the 125-185 kHz VCO and this produces two mixing products.  

For example, let's assume that there is a 40 kHz signal is present on the input that we wish to hear.  If the VCO is tuned 40 kHz above the 125 kHz IF (again, more on that momentarily) - to a frequency of 165 kHz - the switching action of U6A and U6D produces both the sum (165 + 40 = 205 kHz) and the difference (165 - 40 = 125 kHz).

T301 is a filter/transformer that passes only the 125 kHz signal - the difference signal in this case.  This transformer consists of two separate windings, each resonated using its internal capacitors and the externally-added 820 pF capacitors on each winding (e.g. C309/C310) to "pad" it down to 125 kHz.  This forms a fairly wide (8-10 kHz) filter that rejects signals outside the immediate vicinity of its 125 kHz frequency.  Because this filtering is at a fixed frequency, it does not vary with input tuning which means that its bandwidth is constant over frequency.

Of all of the components in this device, this transformer is unique:  It was originally a 262.5 kHz IF transformer from a 1970s/1980s Philco (Ford) AM-only car radio.  While I could have certainly used the original 262.5 kHz frequency - or even 250 kHz, when I built this I decided to pad it down to 125 kHz using C309/C310  - a frequency that is conveniently 1/4th of the 500 kHz resonator.

It's been so long since I built this, I don't recall why I didn't simply divide the 500 kHz by two and readjust that transformer to 250 kHz.  Practically speaking, I could have also up-converted to 455 kHz and used either transformers or ceramic filters from a modern AM radio as 455 kHz ceramic resonators were certainly available at the time - but I didn't do that.

Each half of T301 has a center tap and to this, a bias voltage is applied via R315 to assure that the voltage on these switches was in the middle of the supply range, away from the protection diodes on the 4066's I/O pins, which could cause clipping/distortion should they be allowed to conduct if the signal voltage got too near the ground or supply rails.  To prevent coupling between the two halves of the transformer via the center tap, R314/C311 was added, the resistor adding isolation with the capacitor bypassing the remainder of the signal.  Practically speaking, being able to adjust the bias voltage was unnecessary as a simple resistive voltage divider to set the bias at 2.5 volts (1/2 the supply voltage) would have been just fine.

On the "other" side of the transformer is the other half of U6 (e.g. U6B/U6C) - this time, clocked from the fixed 125 kHz oscillator.  From this, the signal - previously converted up to 125 kHz is now converted back down to audio.

Post-mixer amp/LPF:

The output of the down-converting mixer is applied to U7B via R316, a 1k resistor and a 0.001uF capacitor, both of which form a simple R/C low-pass filter to attenuate any high-frequency leakage signals from the mixer.  Because the mixing process itself is a bit lossy (about 25% efficient) as is transformer/filter T301, U7B boosts the signal by a factor of 10 (20dB) and then applies it to U7A, which is configured as a variable gain amplifier section.  The output of this is then boosted again by U8, an LM386 which is capable of driving headphones or even a small speaker.

A few comments about the design:

Originally, the circuit lacked U7 at all, but it was added when the gain of U8 (the audio amplifier), by itself, was found to be inadequate.  Since U7 was "patched" into place, this explains the odd gain distribution:  If I were rebuilding this from scratch, I'd certainly not need two post-mixer amplifier sections and I could have likely eliminated one full dual op-amp package.  As it is, I may add a "high/low" gain switch somewhere around U5 to allow reduction of the gain somewhat when in the presence of possibly-high ultrasonic signal levels to prevent clipping prior to the band-pass filter which would surely degrade overall performance.

If I were to build this again I would likely use a 455 kHz IF, instead as noted.  While not as plentiful, 455 kHz ceramic resonators are available to use for the BFO as are either transformer or ceramic-based band-pass filters.  I would also likely reconfigure U4B or U5 to perform the high-pass filter function rather than using harder-to-find inductors.

Again, I built this unit in the mid 1990s and have since lost my original notes, but I do recall that I modified it a few times since, simply tacking changes onto the old circuit rather than completely revising it.

Use as a longwave receiver:

While primarily intended to "hear" ultrasonic sounds such as those produced by bats, insects, leaking pipes, arcing power lines, etc., it is just a longwave radio receiver connected to a microphone:  If one connects a few 10s of feet/meters of wire to to J1 - and provides an Earth/ground reference to its shield connection - one can easily tune in the high-power transmitters used for submarine communications (around 20-30 kHz) plus the WWVB time signal at 60 kHz.  This must, of course, be done away from man-made noise sources such as power lines.

Alternatively, I have used a loop of about 1 foot (25cm) diameter of a dozen or so turns of wire along with a 10uF capacitor in series (to optionally block DC from R301) and been able to hear such signals - even in suburbia - but with this arrangement you'll also likely hear plenty of similar signals from the myriad switching supplies that likely inhabit your house as well!

Final comments:

The reader should be under no illusion that this is an optimized circuit or that I would do it this way again:  It was assembled fairly quickly to suit a need and to test a few random ideas, just to see if they would work.  Will I rebuild it at some point?  I don't know - it works as it should, so I don't plan to re-make something that is currently fit for purpose.

While I've heard very few bats with this - probably due to the deficiencies of the electret microphone at ultrasonic frequencies (which explains the future switch to MEMS-type microphones) - I've used it to find powerline noise (arcs are noisy at ultrasonic) and to test longwave receive antennas.

This page stolen from ka7oei.blogspot.com

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