Improving the Selectivity of the R-105A Receiver

I discussed my experiences getting my ARR-15/R-105A receiver on the air in a previous blog posting (here). As I mentioned in that posting, the selectivity is very broad -- so broad that, when using the receiver on, say, an 80 meter AM net, it really suffers from adjacent SSB interference.

I would like to eventually pair this receiver with an ART-13 transmitter I'm working on, but first I needed to improve its selectivity.

How best to do this?

The R-105A has a variable IF -- that is, the IF frequency varies from 450 KHz to 550 KHz, with it being 500 KHz at the BFO "detent." Interestingly, when in MCW mode (i.e. AM), the BFO is OFF when the BFO dial is in its detent ('0') position, but it turns ON as soon as this dial is turned away from its detent.

So, clearly, for AM operation the BFO dial should be left in its detent position (otherwise the BFO is ON and you'll hear it hetrodyning with the carrier).

OK -- so the IF frequency is 500 KHz when copying AM. I wondered if I could cobble a 500 KHz AM mechanical filter into the radio. But first, I needed to find a filter...

I looked around the internet to see if I could find one for sale. Finally I found one on the Fair Radio site (Lima, Ohio). Actually, I found two:

The first filter was contained within the AF Audio Amplifier Module (Fair Radio p/n 546-6053) for the Collins 618T HF Transceiver, and it was a 6 KHz wide, 500 KHz mechanical filter (F500 Y60 (526-9378)). The price Fair Radio was asking for this module, including filter, was $52.00.

The second filter was contained within the 500 KHz IF Audio Amplifier module (Fair Radio p/n 105-AA) for the RT712 / ARC-105 HF Aircraft Transceiver. Fair Radio doesn't specify the filter model number, except to say that it's an AM filter, and that the IF frequency is 500 KHz. The price for this module was $44.00.

(Fair Radio provided the latter with a copy of the module's schematic. I don't know if they also provide this if you order the first module.)

Fair Radio also mentioned that the ARC-105 is a pressurized version of the 618T2 transceiver, so I thought, what the heck, it probably has the same filter as the 618T series transceivers, which is the F500 Y60. And best of all, it was $8 less than the other module.

So I ordered it. When it arrived, I noticed that all of the boards within the module were covered with a clear conformal coating. This wasn't a real big deal, but it's probably one reason why this module costs less than the 618T module. And it kept me from determining what the filter part number actually is, because the conformal coating effectively glued the shield bracket to the filter, and the filter's label is under it.

Here's the module I received from Fair Radio (with its cover removed). The filter is the long cylinder in the upper corner...


Because of the filter's insertion lose, I would need some sort of amplification to compensate. I first tried using the amplifier built onto the filter module's PCB. It actually worked fine, and I initially considered using this PCB with filter and amplifier already built and working for my project, but the board itself is a bit too large for mounting within the R-105A chassis, and so I decided to roll my own amplifier.

I designed and bread-boarded the filter and amplifier circuit and, after some changes, I mounted them on a small piece of copper-clad PCB material. Here's the finished circuit, installed within my R-105A:


This design uses an existing hole within the receiver for mounting, so a purist can, at a later date, easily remove my modification and return the receiver to its original condition. I connected this circuit into the receiver's existing circuitry by unsoldering a wire from one of the pins of the IF transformer Z-119 and soldering in new wires in its place (and connecting another wire to the receiver's AVC line). Again, the new wires can be easily removed and the original wire reinstalled to return the receiver back to its original condition.

Here's a comparison of the receiver's bandwidth with the filter in and out (I've raised the filter's trace by 10 dB so that you can more easily see the difference).


(The above measurement was made by driving the receiver's antenna input with a wideband noise source (General Radio 1383 Random Noise Generator), and then, using a FET probe attached to the grid of the first IF amplifier, capturing the frequency response on my HP 8568B spectrum analyzer).

Here's the schematic:

(Click on image to enlarge)

Notes:

1. I don't have a data sheet for the filter. I'm leaving its drive impedance high (it's the first mixer's load, which, when the filter is switched-in, is simply receiver's IF transformer, unloaded). And I've set the filter's load impedance to 2K ohms. Correct? I've no idea.

[Update, 10 Jan 2010: I just came across the following schematic in Service Bulletin 2 for the Collins 51S-1 receiver in which they install a F500 Y60 filter into the 51S-1. Note that the newly-installed AM filter (bottom half of page) uses a total of102 pF (51 + 51) at the input of the filter and 113 pF at the output (51 + 62). Source impedance is a 10 mH inductor (coupled to filter via 1000 pf) and load impedance is 220K ohms (coupled via 470 pF). I've experimented with using a 10 mH inductor that's switched-in (via the relay) to be the load, in lieu of using the existing transformer (as I'm doing now), and I've also experimented with different values of filter input and output capacitance. When I varied the capacitance I really couldn't see any significant change in filter shape/symmetry, and different 1sy Mixer loads produced different gains, but...nothing else of any real significance, so, at this time, I don't plan to change my design. But I'm including the Collins schematic below in case you'd like to experiment, using it as a basis for your design...]

(Click on image to enlarge.)

2. I installed variable caps (and some additional capacitance using silver-mica caps, similar to what is used at the filter input and output on the RT-712 module) at the filter input and output of my circuit, but during my testing I couldn't really find much difference in filter shape and symmetry with the caps installed or not installed. So I removed the additional silver-mica caps, but left the variable caps (which were peaked to give max response). These could probably be removed, too. The schematic reflects the current implementation that uses only the variable caps.

3. Because of the conformal coating on the PCB, you need to remove the filter and its shield bracket together. But this isn't too difficult. I simply clipped the bracket's four mounting tabs flush to the PCB with a pair of diagonal cutters. The stubs of the tabs that remained on the bracket were sufficiently long enough to allow me to easily solder them to my copper-clad board to hold the filter in place.

4. The relay came from the RT-712 module, too.

5. The filter is switched into the circuit when when the receiver's front panel Channel switch is switched to Channel 10, and only Channel 10. For all other channels the filter is out-of-circuit, and the receiver's selectivity is back to its original bandwidth. (Note: The filter could be switched-in for more channels -- simply connect the additional channels at the Channel switch using individual diodes in a wired-OR wiring.

6. The pot is set to give the same AVC voltage, for a given signal, when the filter is in-circuit, compared to when it is out-of-circuit.

7. [Note (7 January 10): I changed the design slightly (raising the source impedance for the source feeding the filter, lowering the gain of the first transistor stage) from when I first published this post, so the photo above of the implementation doesn't exactly match the schematic, and the schematic revision is Rev B, not Rev A. The following discussion pertains to this new revision]

I first designed the amplifier with only one transistor, but when I tested it I found that there wasn't quite enough gain, so I added the second transistor. I didn't rebalance the gains when I added the second transistor, so the first transistor supplies the majority of the gain (41 dB, assuming an Ic of 0.9 mA and a load of 3.9K || 100K || 33K (Av = gm*Rl)). The second transistor is variable gain, and in my case the pot is set to 3.8 Kohms, so the gain of this second stage is about 9 dB (Av = Rl / Rf). Therefore, the overall gain calculates to be about 50 dB.

50 dB is a lot of gain. I don't know why it needs so much. Hmmm...could I have made a math error in my gain calculation above?

Nope. I just simulated the amplifier using LTSpice IV (a great program, available here for free), and at 500 KHz the gain is 49.3 dB in the simulation.

Why so much loss? I don't know, but...I don't plan to investigate any further: the mod works to my satisfaction, and I've other projects to work on!

8. You can find schematics for the 618T series HF transceivers here.

9. And, as a reminder, my earlier post on the R-105A is here.


And of course, the following always applies...

Standard Caveat...

I hope you find this information useful, but please, use these modifications at your own risk -- although they worked for me, I cannot guarantee that they'll work for you. (After all, I could have made a mistake in transposing them from my lab notebook to this post.)

If you do find any errors, or if you have any questions, please let me know. Thanks!


Note:  November 2, 2013:

I've just received a very helpful note from Cliff, WB6BIH.  He says: 

There is no reason for needing extra gain to compensate for the mechanical filter.  I fear that you may have had a defective mechanical filter.  I bought a few of these several years ago, and found one that I had marked "bad" and it was the SSB filter. The wide AM filter I have works fine.

This is all you need; a .01 from the top of the mixer plate to the filter and I used a combination of disk capacitors of about 120 pf to resonate the filter input coil.  At the grid of the first IF I used a 100 pf mica (that probably would have worked fine for both).  There is more than enough overall gain after a second alignment using a Simpson 260 on the AVC voltage as an indicator. The shield cover goes on over this.  Also, larger capacitors on the last two audio stage cathodes will gain quite a lot more signal.  I will dig into that later, mostly to remove all the odd and useless controls and other connections to switches that only keep it from working.

I worked for the Navy before I retired and somewhere I have the Collins source control drawing on these filters.  I will send you a copy if I happen to come across it.  I spent a lot of time trying to get a smooth passband for these mechanical filters by varying the load and source impedances, but I don't think it matters (helps) much.  Just resonate them with about 110 pf in and out. 

This thing is the darndest technical oddity I have ever seen that you could call a "radio" and I appreciate your notes on getting started. 

Thanks very much, Cliff.  This is a simple mod, and I'll need to give it a try!

- Jeff, K6JCA