Monday, March 31, 2014

Examining the Glencom VC510 UHF to L-Band Upconverter

This is a curious little device - of which several have fallen into my hands.

Often available on EvilBay for fairly cheap, these are in some nice, die-cast Hammond (tm) aluminum boxes approximately 7.25"L x 4.5625"W x 2.125"H (185mm x 118mm x 54mm) in size with two good-quality "N" type connectors connected with short lengths of UT-141 PTFE cable and an board-mounted "F" connector.

The question that seems to be asked by others who run across these devices on the GoogleWeb is "What are these for?"

Well, I can answer that.
Figure 1:
The case of the VC510 "Upconverter"
Click on the image for a larger version.

From the early 90's and into the mid 2000's Hughes Network Systems had a VSAT (Very Small Aperture Terminal Satellite) product referred to as "ISBN" - and an early version of this was called the "Type 2" with much of the hardware being made by NEC in Japan.  Connecting the rooftop satellite transceiver - typically operating in the U.S. market on the Ku band - to the indoor data interface unit was a single coaxial cable that carried not only the power, but all of the myriad control signals used for transmitting - but also the entire 500 MHz of the satellite passband.

Now, you would think that, like everything else satellite that the receive signal would occupy the "L-Band" range of 950-1450 MHz, being down-converted from 11700-12200 - but you would be wrong.  For various and sundry technical reasons, the receive signals were conveyed on the cable from 1000 to 500 MHz - "upside-down" owing to a "high-side" local oscillator within the rooftop unit itself, making it incompatible with L-band gear.

Except that NEC/Hughes had thought of that:  They'd handily included a simple converter within the unit that, using a 1950 MHz oscillator, converted that "upside-down" signal to the proper 950-1450 MHz L-band signal again.

Except that it didn't really work all that well.

Figure 2:
The circuit board of the VC510.  There is also a version that has a
surface-mount 74LS parts instead of the DIP parts shown that is
(pretty much) electrically identical in all other ways.
Click on the image for a larger version.
You see, at about this time, digital signals - data, voice and video - were starting to appear on the satellite bands and this built in L-band converter - while adequate for wideband analog video signals was too unstable and inaccurate for digital signals so the device pictured above was devised to fit the bill, doing what the built-in converter should have done correctly in the first place!

Dissecting the VC510:

Essentially, the VC510 does the same thing as the converter in the original NEC unit should have done:  Mix the 1000-500 MHz signals with a 1950 MHz local oscillator to yield a stable, clean 950-1450 MHz L-band signal - but how did they do it?

To answer this question, I decided "reverse-engineer" the board and came up with the diagram, below.

Figure 3:
A reconstructed circuit diagram of the VC510.
The component designations are arbitrary and are not marked on the board anywhere but with a a circuit this straightforward, it should be pretty easy to work out what's what!
Click on the image for a larger version.
How it works - The frequency converter portion:

The signal from the rooftop unit is coupled via the "line sampler" - a stripline directional coupler etched onto the circuit board that also extracts a bit of the DC power from the coaxial cable as well:  This directional coupler has a negligible effect on the signals passing through it.

From this directional coupler is an elliptical-type low-pass filter that removes signals above approximately 1000 MHz (there may have been a signal at around 1350 MHz - I don't know this for certain) and is amplified by U1 by about 12dB which is then applied to U2, an RMS-11X doubly-balanced mixer which causes a loss of approximately 7 dB.  Mixed with the 1950 MHz signal from the local oscillator the output is passed through an attenuator and then another low-pass filter with a cut-off frequency of approximately 1800-2000 MHz and then amplified by U3 by for another 12dB gain which is the L-Band output.

The local oscillator:

Q1, an AT-41511 transistor along with varactor diode D1 forms a VCO, the output of which is coupled via an attenuator pad to U4, a MMIC that amplifies the signal by 10 dB - some of which is siphoned off and applied to U6, an MB506 divide-by-256 prescaler that takes the 1950 MHz signal down to 7.6171875 MHz (when the PLL is locked) - while the remainder goes to U5 to be amplified again and applied to U2, the RMS-11X mixer.

The main reference oscillator is based around a 7.6171875 MHz (approximately!) crystal, using a 74LS00 NAND gate and fed to a pair of 74LS74 D-type flip-flops wired as a "charge-pump":  If the frequency is too high, a bit of charge is subtracted from C28 and added to C29 and vice-versa if the frequency is too low. U9, a TL071 op amp which is used as a loop filter/integrator and does the phase/frequency control, locking the VCO to the frequency reference provided by the crystal.

In all, there's nothing about the above circuitry that is particularly fancy or requires exotic components - just the application of fairly inexpensive, standard components using designs that had been around since the late 60's or early 70's - except, perhaps, for U6, the prescaler.

  • U1 and U2 are very similar to the MSA-2086 (but a different package) and good from DC to at least 2.5 GHz and typically have 10-12 dB gain over this range and a 6-7 dB noise figure with a 1dB compression power output of around +4dBm  The typical bias current is 25 mA with 5.0 volts at the output terminal.   This device is generally equivalent to the Mini-Circuits MAR-2.
  • The MSA-1105 used for U4 and U5 is good from below 50 MHz to 1300 MHz at the -3dB points with a typical gain of 10-12 dB and usable to over 2 GHz with a gain reduction to around 6dB.  Up to 1.3 GHz the 1dB compression power output is typically +18dBm dropping to around +15 dBm at 2 GHz with the noise figure below 1 GHz typically being below 4 dB and rising to around 5.5 dB at 2 GHz.  The typical bias current is 60 mA with 5.5 volts at the output terminal.  This device is generally equivalent to the Mini-Circuits MAV-11.

Testing on the workbench:

Surprisingly, the unit produced a fairly good CW "note" - almost suitable for CW/SSB operation - something that could have probably been cleaned up had a better crystal reference oscillator used.  With no modification at all, the VCO's lock range turned out to be approximately 1600-2150 MHz by varying the frequency fed to the crystal oscillator from 6.25-8.398 MHz - but it could probably extended by modification of the cutting/bridging some traces in the VCO section.

As it is, the "gate-type" crystal oscillator based on the '7400 is not accurate/thermally stable enough for SSB/CW operation - or even narrowband FM operation - so if this sort of operation is anticipated, a different, more thermally-stable oscillator is likely required!

So, what's it good for?

It's hard to say, but some of the ATV folks seem to have found use of these devices as 23cm and/or 13cm ATV converters and in theory it could be used to convert 2 gig WiFi to other frequency ranges or even be the front end of a simple spectrum analyzer for the low GHz range.

Because the RMS-11X mixer is rated for as low as 5 MHz on all ports, up to 1000 MHz on the IF port (to which the F-connector sends the signal) and to 1900 MHz on the LO and RF ports, it should be perfectly usable to at least 2500 MHz - perhaps higher, especially if preceded with a low-noise amplifier.

A few comments about modification:

- L1/L2/L3 are circuit board inductor traces that can be sliced.  If the accompanying capacitors are removed, the low-pass response of this filter is eliminated and useful response is extended well past 2 GHz.

- L4 is a circuit board inductor and its low-pass response is also eliminated if its accompanying capacitors are removed.

- If the L1-L4 filtering is removed, additional (narrowband) filtering for the frequencies of interest should be added to the input and output to prevent/minimize spurious responses.

- As noted on the schematic, there are also some traces that could be sliced/jumpered in the VCO section.  It is likely that modification of these could change the VCO tuning range from that noted above.

Please note that the rating of the prescaler, MMIC amplifiers and the mixer would limit the upper end of the useful range of the VCO to something in the 2.2-2.4 GHz range at most, but it should be possible - in theory - to take the VCO down to well below 1 GHz with the addition of a physically larger inductor.  If this is done, one might want to rewire the prescaler as well to give a different divisor ratio (e.g. divide-by-128 or even divide-by-64) using the information on the diagram.

Now you know!


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