High Performance AGC Amplifier

26 Jan

I’ve been looking for a high performance AGC amplifier. Some criteria are:

  • Wide control range with linear volts/dB.
  • Strong input signal handling capability
  • Decent noise performance

For a while, I thought the HyCAS amplifier developed Hayward et al was it. However, some LTSpice modelling followed by bench analysis indicated that the amp suffered from very non-linear gain control as well as completely collapsing with strong input signals when gain is set a minimum. This is because the circuit reduces gain by starving the input JFET stage of drain voltage. With maximum gain reduction, the FET has virtually no voltage to work with. That’s a no-no.

I developed an enhanced version of the HyCas that linearizes the gain control over a 70 dB range and suffers no loss of input dynamic range. It does this by varying the JFET  source voltage, driving the FET current towards pinchoff and decreasing the current. So far, it’s only been run as a LTSpice model, so it needs validation. Therefore, I present the circuit solely for your edification until I build it and test it. The amplifier in the schematic above is tuned for 9 mHz and has about 14 dB gain, but it can be re-tuned for most any frequency in the HF range.  The resistor across the tuned circuit is to limit the gain and prevent oscillations. The diodes on the control voltage, together with the input resistor,  linearize the control range, otherwise it gets way too sensitive at the bottom end of the control range. This is very important in AGC applications, as severe non-linearities can cause motorboating and other anomolies. The linearization of the control voltage also makes it much more useful to develop an S-meter from this voltage.

If  you decide to build it, keep in mind that it was developed for a U309 (J309 is identical), so it will perform differently for a more common J310. Also, JFETs can vary widely with respect to their pinchoff voltages, so some tweaks to the linearizer might be required.


2/1/2018 Update:

I got an LTSpice model for the J310 and confirmed it’s a different beast in this circuit. I ordered a bunch of ‘309’s and will report when I get a chance to try them in a real circuit.





QER Crystal Filter Designer

31 Jul

The QER filter is a special case of ladder filter that has a very smooth passband response and is very scalable just by adding additional center sections. All capacitor values are equal and it isn’t as termination sensitive as some other filter typologies. Note that the extra paralleled crystals on the end sections don’t count as filter poles, so a 4 pole filter has 6 crystals. I find that 6 poles are really necessary for good sideband rejection. This is a good rule of thumb and isn’t peculiar to this filter topology.

A simple way to build up a filter is to design a 2-pole unit (no shunt crystals, 1 shunt cap) first and get it working. Since all caps are equal, you can start off with arbitrary cap values, say 50-80 pf fo an SSB filter, proportionally higher for CW. For development purposes, put a 500 ohm pot in series with each of your 50 ohm test equipment outputs/inputs and initially set them to mid-way. Adjust the pots for minimum ripple, or better yet, best match in the passband, then measure the pot values. This is your desired termination impedance. It will be higher for an SSB filter and lower for a CW filter.  Lower or raise the cap values for wider or narrower bandwidth respectively, re-adjusting the termination resistor values as you go. Be aware that the filter bandwidth will shrink as you add xtal/cap sections. so start off with a bandwidth about 1.5 x what you want to end up with. This matching process using variable resistor terminations will have large filter loss during development (25-30 dB), but you’ll get it all back when you match the filter impedance to the circuit load using either a transformer or LC networks. Add xtal/cap sections one at a time until you get the filter complexity and performance you’re looking for.

Here’s a simple 4-pole, 2 kHz BW design. Note: If you build this design, use your own measured crystal parameters. When I analyzed this 9 Mhz design, I used the motional parameters from a 6 Mhz crystal I had data for.

For more theory about crystal filter design, see this page on this site: Crystal Filter Design Simplified



The LTSpice design file can be found here. After downloading, start LTSpice, then navigate to the file. There are notes in it that make it pretty self-explanatory and easy to use.

The “QSSR” Single Signal Phasing Receiver

15 Sep

I’ve always been interested in phasing-type DC receivers. The recent spate of software-defined radio (SDR) projects based on the Tayloe QSD detector got me wanting to build a phasing receiver based on a hardware implemenation, but using the QSD. I have a prototype running on my bench and it looks very, very good and is easily made to work on any HF band merely by using the correct pre-selector filter. It easily achieves well below 0.3uv sensitivity across the range and has an AGC’d front-end for gain control and antenna isolation. I’ll be unfolding this here in the next couple weeks.


Radio displayQSSR


Updated 1/24/2016



Slight Course Change on X1M AGC

29 Jun

Been pre-occupied with other priorities lately, so haven’t spent much time on any of my ham projects. This past weekend was a rain washout here, so I got a few hours to get on the bench and play around with the X1M project.

As I expressed previously, it was my intention to embed an Arduino Nano inside the X1M and use it to (among other things) implement a firmware controlled AGC process. After actually embedding a Nano and seeing how cramped the installation is, reality set in and I started considering another approach. I’m now thinking more along the lines of a ‘back-pack’ unit containing the Arduno and other enhancement circuits, connected to the main radio via the existing DB9 connector. The DB9 presently has about 1/2 dozen of its pins connected to points inside the radio that I don’t see myself ever needing. My plan is to re-purpose these pins so that the functionality I want to implement can be accomplished in a less intrusive, more flexible way outside the box. Also, I’m scaling back my ambitions (for now) and am planning on putting a hardware AGC board inside the radio, but bringing out the AGC voltage for the Arduino to measure and turn into an external software-calibrated S-meter function.

In the process of looking at the best way to do all this, I think I discovered a design flaw in the X1M having to do with gain distribution. It turns out that the SA612 product detector overloads on strong signals about 15dB before any of the previous IF stages and the following AF stages, becoming a bottleneck for strong signal handling. If the radio had AGC, this might not be so important, but in a fixed-gain radio it’s very important to have as much strong signal capability as possible leading up to the first gain-control element. In this case that’s the volume control, at least when there’s no AGC add-on. I think this problem is due to the use of the MC1350 as an IF amp which, in my estimation, has far too much gain for most IF applications. I plan to look at this some more to determine if my reasoning is correct, but I might be suggesting an IF gain reduction mod, even if AGC isn’t added.

Anyway, look forward to a change in direction as I implement the AGC and add the external Arduino.


Embedding the Arduino Nano Inside The X1M – Part 1

9 Jun

This weekend I got some time to try out my concept for embedding an Arduino Nano inside the X1M. You can find more details here.

X1M Transceiver in the Field

4 Jun

I just got back from a 5 day camping outing where I got a chance to try out my X1M field kit. You can read about it here.

My X1M Transceiver Field Kit is Ready

26 May

I finally got all the pieces of my X1M field kit built and tested. I’ll be going out for 5 days camping in my motorhome next week, so I plan to spend some time evaluating the whole setup.

The kit consists of the X1M Transceiver, a 7AH battery, a homemade slingshot antenna launcher, an EFHW wire antenna supported by 0.07″ dacron cord, an antenna tuner/SWR indicator an audio interface box and any one of my Android mobile devices for software/rig control. For now, I’m using an ASUS Transformer (because it has a keyboard) and the DroidPSK & Pocket RxTX apps available from the Google store.