IF Output for the FRG-7700

I'm in the middle of building a new workbench for myself - well, the actual reason is to build my partner a study desk, but she's let me attach my workbench to it - so actually doing stuff has been a bit slow.

One of the reasons I decided to get back into SWL was to receive DRM signals. Back when I drifted away from SWL, DRM was just beginning to appear locally - RNZI had some broadcasts and, from memory, so did Radio Australia. Since there were practically no DRM receivers available, software decoding was (& still is*) common - the intermediate frequency (IF) of a normal radio was tapped off at a suitable point, fed to a mixer/downconverter (to produce a 12kHz IF signal), and sent to the PC soundcard for decoding.

Being a good solid communications receiver, the FRG-7700 is a pretty good (if not great) radio for DRM & other digital modes. Traditionally, picking up the IF at one end of VR1002 is usually suggested - it's a nice low impedance point (so the mixer's reasonably high 1.5k input impedance doesn't noticeably affect the signal), and the signal level there is just under ~300mV, which is pretty much perfect for a typical NE/SA-602/612 mixer. It works, and works reasonably well - the only problem is that it's after the IF filters. Even in the AM-Wide position, the nominal bandwidth of the filters is ±5.5kHz, or 11kHz total - j-u-s-t enough for the usual DRM signal (typically 10kHz BW on shortwave) to squeeze through, especially if you ignore that the filter bandwidth is specified at the -6dB points...

Despite that it worked OK, and I regularly decoded RNZI's DRM broadcasts with that connection. However I knew it could be better, so I took a good look at the circuit and main board around the 455kHz IF strip. The first thing I noticed is that there's very few places to get at the signal without lifting the main board - either by design or good (bad?) luck, all the easily-accessible component leads are tied to ground. R95 - between Q1022 and the filter bank - is about the only point that's accessible. Secondly, everything around there is middling-high impedance, and the signal level is low. The best idea seemed to be to build a small high-impedance buffer amp...

So I came up with the circuit shown (which I've re-created in LTSpice from my hand-written notes). Being made from basically leftover parts it's certainly not an optimum design, but it did what it needed to - the JFET input is high impedance (several megohms at least), the bipolar output is fairly low output impedance (I never calculated it, but it's low enough to easily drive a SA602 mixer), and it provided enough gain to match the level from VR1002. A few back-of-the-envelope calculations suggested I'd need ~20dB of gain, and in the end the completed amp had a gain of ~23dB. SPICE modelling of the circuit indicates the gain should be ~29dB. Hmmm...

455kHz IF amp for FRG-7700

The semis are fairly common and not at all critical - the BF245A could be any small signal JFET, and the BC547's could be replaced with anything similar e.g. BC549, 2N2222A (which would probably result in higher gain; not a great problem). Ideally, C1-3 would be something like 0.047uF, not 0.01uF - but again, it's not critical. Another thing that's not critical is layout - 455kHz is practically audio, not RF - and the completed circuit was built on a strip of cheap veroboard fitted between the main board and rear chassis of the radio on a couple of adhesive standoffs. The input was connected via shielded cable and soldered to the 'top' of R95. Since my radio didn't have a memory unit a BNC socket for the IF output was mounted on the rear panel memory unit access cover, and 11v power was taken from connector P16.

(Which brings up an interesting point I rediscovered recently and indirectly reminded me to post this circuit. According to the circuit diagrams I have, P16 should have 11v on pin 2 and BU13.5v on pin 3. In my set, these are reversed. Anyone with a working memory unit care to comment?)

IF amp installed

The end result appeared to be a fairly substantial improvement in DRM decoding, although I never made any quantitative measurements. It certainly seemed that signals which previously were borderline could be received & decoded with ease. Unfortunately, not long afterwards I packed the radio away - but not before pulling the amplifier and donating it (along with my IF downconverter) to a friend of a friend. As far as I know both are still running.

Which brings me to the now. While I'm finishing things off I've been looking at the circuit diagram for the memory unit. It looks complicated, but it's actually remarkably simple - basically, when the "M" button is pressed an analogue frequency counter converts the VFO (main dial) frequency into a 13-bit word, which then gets stored in RAM (in the address set by the memory location switch) alongside the bandswitch/VCO/ front-end settings. When the "MR" switch is operated the VFO in the main unit is disabled, the bandswitch/VCO/front end settings associated with the chosen address are read out and fed to the main unit, and the original VFO frequency is recreated by a VCO in the memory unit. Since mine doesn't have the memory unit ... well, let's just say I'm working on an interface board which will allow an external microcontroller (e.g. Arduino, Minimite, STAMP, etc) to work as a memory unit.

And I think I can squeeze an IF amp & BNC socket onto the board, which will be small enough to replace the existing memory unit access panel...

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(* Yes, I'm aware that many DRM receiver chips have been announced over the years - and some have even gone into production! Just try and get your hands on one - they're aimed at big manufacturers, not the small scale or hobbyist markets. And the big manufacturers aren't building DRM receivers either...)