|Figure 1: |
The front panel of the Drake SP75 speech processor.
"That sounds terrible!"
"Your 'lows' are completely missing!"
"Your audio sounds 'restricted'"
"Turn it off!"
I obliged, of course, but I also knew that in the past I could switch in the speech processor - but set it to a very low level of clipping and no-one could really tell the difference between it being switching in and switched out, so I knew that something had definitely changed!
Later, I used some available test equipment (computers and software) to see what had changed, setting up the following:
- Using another computer (a netbook) I ran the Audacity (link) program, a free, open-source audio editor. Using that program, I generated 5-10 minutes of white noise and set it to play back that white noise as a loop.
- The generated white noise from that computer was fed into the "Tape" input of the speech processor with the audio level set just high enough to properly drive it.
- The audio output from the SP75 was fed into another computer running the Spectran (link) program - also free - which does audio analysis. Another program that would work, but is more difficult to use with a much steeper learning curve, is "Spectrum Lab" (link).
By this point, I was fairly sure that I already knew the answer - and the above technique of "sweeping" the audio passband using white noise verified it: The "low end" audio frequencies (below approximately 700 Hz) were being rolled off significantly - by 6-10 dB and more, explaining why my audio sounded so bad!
How the SP75 works:
Before we go on, a few words on how the SP75 works.
This is a combination AF/RF speech processor and it works by first routing the input audio through an XR2216 audio compressor. Then, the audio is double-sideband modulated at around 459 kHz, filtered to produce a lower sideband signal using a pair of 455 kHz ceramic filters, RF clipped, filtered by another ceramic filter, and then demodulated back to audio. By applying the clipping at RF, the distortion products are (largely) generated out-of-band rather than at audio.
By applying both audio compression - to assure a consistent amount of RF produced for the SSB modulator, and RF clipping, the "best of both worlds" in terms of audio processing can be applied in terms of improving the "peak-to-average" ratio for speech while minimally increasing the amount of perceived distortion.
What had gone wrong:
Because of the loss of low frequency audio, I figured that one of three things had happened:
- One or more electrolytic capacitors in the audio path had dried out and decreased in value causing the loss of low frequency response.
- The BFO, nominally at 459 kHz, had gone off-frequency and caused the audio passband through the filters to shift.
- One or more of the 455 kHz ceramic filters had gone bad.
I then fired up an SDR (Software Defined Radio) - an RF Space SDR-14 - and started probing around inside the SP75, noting that the BFO was, in fact, where it should have been: within a few 10's of Hz of 459 kHz, ruling out the second probability of the above.
Connecting the input of the SDR-14 to test point 11 through a 2k resistor to minimize circuit loading, a location after all of the filtering and clipping, I centered the SDR on the passband - while still sending white noise through the SP75 - and looked at the resulting display and saw that it was anything but flat, indicating that one or more of the ceramic filters in the unit had, in fact, gone bad.
Identifying the ceramic filters:
In looking at the filters themselves they were clearly made by Murata, marked with "CFW455" followed by what looked like an "I6" printed in white ink while the schematic diagram simply called out a part number of "CFW455I". In doing a bit of research on the web, I determined that the Murata "CFW455I" had the following specifications:
- Center frequency: 455 kHz
- Input/Output Impedance: 2 kohms
- -6dB bandwidth: +/- 2 kHz
- Stop bandwidth: +/- 7 kHz (at -50dB)
- This device was a 6 pole filter
The new filter (left) and the old filter (right).
All but one of the leads lines up with the original.
Click on the image for a larger version.
What I did NOT want to do was use "new-old" stock because of age-related degeneration with these parts. Typically, these parts have silkscreened, silver-plated electrodes on the surfaces of their ceramic elements, but even though these are usually fairly well sealed, they gradually degrade for whatever reason, either due to slow corrosion of the potting compound that protects them, ingress of moisture from the environment, or possibly due to electrolytic degradation due to chemical reaction and/or voltage applied to their terminals.
Whatever the reason for their degradation, I decided that I did not want to get "new" 10-20 year old parts and risk having them be out of spec!
In perusing the various catalogs, I noticed that Murata does still make a part that is electrically identical - the CFWLB455KJFA-B0, available from Mouser Electronics, so I ordered some.
Installing the replacement parts:
The trace at the center filter position (FL2) that
inevitably lined up with one of our newly-drilled holes.
Click on the image for a larger version.
As can be seen in Figure 2, the new filter is slightly smaller than the old one - and the pinout is slightly different, as well, but fortunately there is only ONE pin (the "output" - but these filters are bilateral, so it doesn't matter which is used for which) that is actually in a different physical location which means that we need to drill just one hole for each of the filter locations.
Referring to Figure 3, above, you will note that the new hole is in line with the other pin and straight "above" the existing hole, a fact that makes it fairly easy to locate the precise position of this new pin.
Of course, Murphy has to intervene as shown in Figure 4 where the extra hole drilled for FL2 ended up going right through through a trace on the top side of the circuit board.
Fortunately, we have the technology (e.g. soldering iron, solder, wire) to relocate this trace and get around this problem (literally!)
Both ends of the trace that ran under the original filter were sliced with a sharp knife and the original trace was heated with a hot soldering iron so that it lifted off the board. The ends of the trace were then scraped clean of the green coating and a short piece (some #30 wire-wrap) of wire was soldered into placed, used to route around where the filter would be placed as depicted in Figure 5.
The removed and re-routed trace using a short
piece of #30 "wire wrap" wire.
Click on the image for a larger version.
Having done this, the board was now ready to receive the three new filters.
Because only the lead with the drilled hole does not match the original pinout, they may be (mostly) soldered as normal. For that "other" lead, a short piece of wire - a trimmed component lead, for example - may be used to make the connection to the original, now-unused hole to the new pin as seen in Figure 6, below.
On the bottom side of the board, the installed filters and the jumpers to the leads that connect
to the positions with the newly-drilled holes.
Click on the image for a larger version.
Meanwhile, on the top side of the board, the filters look like this:
Getting the SP75 back into working order:
Firing up the SP75 after replacing the filters I noticed immediately that its audio didn't sound right - as in very "tinny", even worse than before. Putting the white noise back into its input and connecting the SDR-14 to TP-11 I noticed immediately that the 459 kHz BFO frequency was entirely outside the passband of the 455 kHz filters.
What had happened?
From what I can tell, one of two things might have changed:
- These new filters (CFWLB455KJFA-B0) are slightly narrower than the original CFW455I ceramic filters used by Drake. In this scenario, the BFO and the edge of the audio passband would have been "moved" entirely outside the filter.
- The original Drake filters were marked "CFW455I6" - a designation that doesn't seem to be correlate with anything in a catalog that I could find. Perhaps the "6" indicates a center frequency of "456" kHz? If this is the case, that would imply that the original filters were specially-selected for the higher center frequency and, perhaps, matched bandwidths. Based on what I was seeing, having the passband of the filter shifted up 1 kHz to 456 kHz would place it in about the right place.
- Slightly reworking the oscillator to use L/C components such as a 455 kHz IF "can" (transformer) as frequency-determining elements. This would, at the very least, involve adding a series DC-blocking capacitor were this route taken. A bit of care would need to be taken to assure that this arrangement was temperature stable to within a few hundred Hz over the expected frequency range.
- Using an inexpensive 455 kHz ceramic resonator - also available from Mouser.
There's no real reason why LSB (lower sideband) must be used when picking the BFO frequency as these filters are symmetrical.
If you use an L/C network for setting the frequency, pick the frequency that gives the best results using the methods described below. It so-happens, however, that ceramic resonators are easier to move up in frequency than down as this requires just series capacitance, so using a "high side" BFO and LSB is just easier in this case!How to determine the correct BFO frequency for your filters:
To determine the correct BFO frequency I used the same method that I'd used to determine that the original filters had gone bad in the first place, that is:
- Insert a white noise source - at just high enough audio level to drive the SP75, but low enough to avoid any overload or clipping - into the input of the processor.
- Using a program like Spectran to observe the audio spectra, note the "flatness" of the audio output taken from the speech processor and fed into a computer.
Because these ceramic filters are considered to be "low cost" they do have a bit of intrinsic ripple (their specifications are for +/2 dB of ripple) and they do not have a "brick wall" response, so don't expect a superior "shape factor" - that is, a very abrupt cut-off, but rather a fairly gradual cut off over the span of several hundred Hz or a kHz. If you are a purist, you can order several extra filters so that you may pick and choose which one(s) give the best, overall response - but note that the circuit board cannot take very much soldering/unsoldering, so you would want to install sockets or some other temporary connections were you swapping filters in and out frequently!
By carefully adjusting the BFO frequency, one should be able to get a fairly flat frequency response down to 200 Hz or so and up beyond 3000 Hz, fully encompassing the frequency range of any transmit audio source that you'd be likely to use!
Consider the result below:
Wrapping it up:
Overall, I'm pleased with the result, even though the project was a bit more involved than I'd expected it to be. Up to a clipping level setting of 6-9 dB, there is hardly any noticeable distortion added to the audio - just as it used to be when the SP75 was new!
Of course, a speech processor is one of those things that should be used sparingly. Under normal conditions with good signals it is probably not needed at all and when conditions start to get a bit rough, the added compression - if not taken to a ridiculous level - should add more "punch" to a signal than it would degrade the audio due to excess compression, clipping, distortion and/or coloration. This particular processor's "clipping level" control goes all of the way to 20 dB - a ridiculous amount that yields results that may be intelligible, but are likely to be unpleasant, so it should never be used in any but the worst possible band conditions - if even then!
It is worth paying very close attention to the SP75 manual in setting up the input and output levels for the SP75 for the microphone that you plan to use.
If the input level is too high, there may be too much audio compression in the XR2216 stage while too low, the efficacy of the processor itself is reduced. Also, the output level control should be set so that the audio level is the same when the processor is switched out (e.g. bypassed) and in, but with clipping set to 0 dB.