Designing a 20/40 band CW rig – Part 4

Hi all,

This is a continuation of an earlier post, which can be found here.

Work has been continuing on my CW rig design, and now I will share another subsystem of the rig. I will get to the audio, but in this post I will be looking at the mixing function.

The SA612

Many rig designs use a SA612 mixer oscillator IC. This device is a double balanced mixer and oscillator. It can actually be used with either balanced or unbalanced RF inputs, and a balanced or unbalanced output can be taken. Regarding the oscillator, essentially only the tank circuit is external. The SA612 can also be driven by an external LO input.

Why not use it? Good question. My early concepts made use of it, but the limitation of the SA612 is its dynamic range. The third order Intercept (IP3) is at about -13dBm with a -45dBm signal. The 1dB compression point is -20dB, but given the IP3 at -13dB, a signal at this level of power would be full of inter-modulation distortion. I wanted something that would give much better inter-modulation distortion performance. In looking around, I found nothing that was quite like the SA612, which is why so many designs make use of it.

You’ll be able to see what I have done in the circuit below. This is a screen shot, with the RX signal coming in from the top from the TX blocking circuit discussed in previous posts. Note you can click on the picture for a zoomed in view:

CW rig mixing circuit

The minicircuits ADE-1L

Minicircuits have a large variety of mixers available. Most service microwave requirements, but some work down to HF. I had a good look at a number, but the ADE-1L took my interest. This device has as 1db compression at 0dBm, which is 20dB better than the SA612, along with a IP3 at 17dBm, 30dB better than the SA612. Dynamic range is therefore effectively 20dB better, and inter-modulation distortion performance should be 30dB better. Using these devices should make for a much better rig. Unfortunatley, there will need to be more work to use one (or two) of these than SA612 ICs. Lets get into dealing with the issues.

50 ohm input and output

The ADE-1L is designed for 50 ohms. The SA612 has a high input impedance, 2k is ok to feed it, while it has an output impedance of 1700 ohms. 50 ohms sounds better, but it is harder to use. What is coming in might be expected to be 50 ohms at the antenna, but by the time it gets through the TX blocking circuit, it is around 10 ohms. Essentially both devices need matching circuits – which is band specific. I discussed these networks more on a previous post.

The first mixer

The the first ADE-1L mixer, M1, takes the RF input and a LO. The RF is 7 to to 7.3MHz or 14 to 14.3MHz depending on the band. If 7 is mixed with 11, 4 is output. If 14 is mixed with 10, then 4 is output. This LO needs to change with tuning, needs to be around 3dBm, and these things can be done by the AD9834. I will look at this device more when I look at the microprocessor control, but it can be used to generate the carrier at 7 or 14 (and up the band) during TX, and generate the around 10 or 11 MHz during RX for mixing.

With these two signals mixed, I have output at 4MHz. I also have an image frequency at 18MHz for 40 metres, and 24MHz for 20 metres. There might be other products coming out of the mixer as well, but these will be well down on the 4MHz and the image frequency. I should not have much RF and LO coming out, given the isolation performance of the mixer.

Some amplification

All I have done with the RX coming in so far is to impedance match it to the output finals (which are like leaky closed switches during RX) pass through the TX block and then match it to 50 ohms for the mixer. The mixer loses 5.5dB mixing, and there would be expected to be 1 or 2 dB across all of the matching networks. There is a little over 1dB across the TX block. All of this is not such a big deal because we are dealing with 20 and 40 metres, and we don’t need to worry about noise performance so far. The SA612 actually amplifies the output as well as mixing it. There is around 5dB of loss in mixing, and then 22dB of gain, leaving a net 15dB gain with over 5dB noise figure (because of the mixer).

I have put a 4401 device to do this amplification, because the ADE-1L is only a mixer. The 50ohm output of the mixer is impedance matched to 700 ohms. This allows a lower current biasing network on the transistor. A BJT is used to keep things linear as they do this job better than MOS devices, especially at these low current levels. The 4401 is designed to give 23dB gain. I use it to do a small impedance transformation, back down to 400 ohms, a suitable level for the crystal filter about to come.

The crystal filter

I have an intermediate frequency of 4MHz with this design. Now using a series of 4MHz crystals, I can have a narrow pass band filter. The great thing about crystals is that they can be used with some series and shunt capacitors to give various types of high performance filters, such as Butterworth or Tchebycheff filters. Butterworth are a little lower performance, but have no ripple. I have designed this for a small amount of ripple (1dB) with steeper skirts, making it a Tchebycheff. The 330pF and 390pF capacitor radio decides the ripple. The amount of capacitance overall decides the pass bandwidth. I have designed for 500Hz. It is a reasonable compromise between selectivity and usability. A SSB filter width is too much for a CW rig. I think 300Hz is too tight, except in contests, but I am not really designing it for contests, more for SOTA activations.

The number of crystals forms the number of “poles”. 4 poles seems to be a good compromise for these filters, and many CW rigs have settled on this number. I will too. After the 4 crystals, I have a matching network to bring the impedance back down to 50 ohms for the second mixer.

The second local oscillator

The second mixer needs another local oscillator, this time pulled just off 4MHz, so that I get audio out. If I was using a SA612, I just need a tank circuit, but here I need something more. I need an oscillator, plus I need to get the LO up to 3dBm for the ADE-1L.

I spent quite a bit of time on this circuit. One of the problems is that the crystal can make the input to the first transistor quite free of harmonics, but the output is not much so. I found that it worked better with quite a high impedance biasing network. I also have gone with a Clapp oscillator fed from the emitter of the active device. The output has a collector resistor, but no inductor. This allows a moderate impedance path for the harmonics to go. The desired output goes through a series resonant circuit, to pass the fundamental, and then a parallel resonant circuit to shunt any remaining harmonics to ground. Most of the harmonics leave through the collector. The approach works quite well. I then use a second active device to bring the oscillation up to 3dB and impedance match to 50 ohms. Again, I have no inductor on the collector, so any harmonics (there is still a small amount) are shunted to AC ground.

At the end of all that, I have a near 4MHz local oscillator, controllable through a varicap on the crystal, mixing with a 4MHz intermediate frequency. This will yeild audio frequency output.

There will be, of course another 5.5db loss across this second mixer, so I have a net -5.5 + 22 – 5.5 for 11dB gain across all the mixers. S9 has gone from about -70dBm to 59dBm.

Next up is the audio circuit, which includes a automatic gain control. I’ll look into that for the next post.

Regards, 73, Wayne VK3WAM

Portable preamps for 6m, 2m and 70cm – Part 2

Hi all,

This is a continuation of a post at: Portable preamps for 6/2/70 part 1.

Part 1 showed a schematic for the preamp component. This would live on the antenna, near the feedpoint. Given the bias tee, it will only need to be fed by coax. It does not need any control lines or separate DC cables going up there.

Here is a picture of the PCB design:

Preamplifier board

I’ve got this under 2 inches by 1.5 inches, so I am happy with the size. I have two BNC connectors on board, these can be obtained from rfsupplier. I used microwave design techniques on this board with as much as possible of the RF being microstrip with 50 ohms characteristic impedance. Of course, with this approach, there is no using vias for anything apart from ground connections. In line with Minicircuits recommendations, the PGA-103 is surrounded with grounding vias. I also ensured that there were grounding vias near each of the relay RF inputs and outputs.

Sequencer design

The sequencer needs to do several things:

  • Provide the Vdd supply for the preamp and put this on the coax
  • Only put Vdd on the coax when the radio is Rxing – otherwise the preamp will be exposed to RF far beyond spec, and magic smoke will not be far behind. As the preamp uses non latching relays, any removal of DC from the coax will cause the relays of the preamp to switch to bypass, allowing TX RF to safely pass.
  • Allow for a delay between the radio commencing TX and high power RF going out the cable. Modern full mode rigs have TX inhibiter inputs, and the FT-817, 57 and 97 are no exceptions. The microprocessor can respond in a few microseconds, but relays need a few milliseconds to switch. The plan is to allow 25 milliseconds to be safe.
  • Also provide for a short delay when TX stops before switching back to RX. This would be about 10ms.
  • The local bias tee to put the DC on the cable needs to withstand 100 watts ssb. This will mean this bias tee will not need any relays to switch the RF. Of course there are relays in the preamp at the antenna end of the coax, where they are needed.

Microchip manufacture the PIC range of controllers. The task needed here is basically a timer and a state machine to deal with the various scenarios. Others who have built sequencers have used PIC controllers. In a previous life, I wrote real time control systems on PCs. The requirements here are far more simple! Looking at the range, the entry level PIC10 series will do the job. While not in the data sheet for the device, the Microchip web site also details that this device has current source/sink capability of 25mA. This is quite nice, and with a 5V supply, should give lots of drive and sink options.

On the FT-817 and 97 there is an ACC port. This has:

  • TX GND (a sink when the radio wants to TX, otherwise floating – i.e. open collector),
  • TX INH signal. This input feeds the base of a BJT through a 47k resistor in the radio. The transistor needs to sink 8V through a 3.3k resistor to prevent TX. This is 2.4mA. We need to provide 1/Beta of this current, and Beta should be 40+, so only 53uA or so is needed.
  • A 13.8V (or actually whatever the voltage that is feeding the radio is – so on a FT-817 this could be 9V for instance. There is a 10ohm 1/10th watt resistor on the radio in series with this source, so this source is practically limited to about 80mA. We will not want to power things from this, but we can present high impedance to it and use it as a signal to indicate the presence of the radio.
  • A GND

Here’s a look at the circuit:

Preamp sequencer for 6/2/70

EDIT: This diagram is updated to correct a small drawing bug.

I thought about whether I would use a transistor to switch the 5V supply to the bias tee and onto the coax, but there could be 300 to 400mA being pulled when on, and this is a bit to sink when it needs to be off. Instead, I introduced a relay to switch the supply voltage to the bias tee.

There are actually two bias tees. One for 6m and the other a compromise between 2 and 70. Both the FT-897 and the FT-817 have two antenna connections. The 897 is not selectable, one is HF/6m and the other is 2/70. I have largely gone with this configuration, however the 6m optimised tee could drive 2m. The FT-817 is fully switch-able, so either of its antenna connections can be used for any band. I have selected bias tee components that can be switched, so one of these circuits could have the left optimised for 2, the other for 70, or have both bias tees the same configuration. Again, I selected bias tee components to sink the fraction of power that will go through the tee from TX power at 100 w ssb, CW or 50w PSK. This is actually one of the key reasons for two inductors on each tee. One is a ferrite coil, the other is a chip inductor. Both are from Coilcraft and the part numbers are going to be quite specific. It will not be possible to use any inductor of a similar inductance value, because the actual inductance at the target frequency can be quite different to expectations. There is also the need to actually sink some of the power that comes through here as inductors are not known for having high Q – Q effectively being a ratio between inductor resistance and reactance.

The relay needs around 12mA to drive the coil. I am using two 4401’s with the coil on the collector of the first. Q1 has a PIC output on the base. If this is taken high, then Q1 will conduct. 2mA will be drawn from GP0 on the PIC, well within the 25mA limit. Q2 has the radio 13.8V output on its base. The resistor is sized again to provide for a 2mA draw. If the radio is connected, then Q2 will conduct, otherwise not. This setup is effectively and AND operation. Both the PIC output and the radio 13.8V signal (which just needs to be any meaningful non-zero voltage) are required for the coil to switch from NC to NO and provide the bias voltage onto the bias tees. No radio connected to this sequencer, then no Q2 conducting, and therefore no DC bias on the coax. This results in the preamps at the other end being safely deactivated and bypassed.

GP3 is an input only pin and it is connected to TX GND. This is an open collector pin, acting as a current sink when activated, with very low resistance. About 1mA will be pulled out of the GP3 chip when low. The PIC has TTL inputs, and also provides weak pull-ups, so no external pull-up resistor between TX GND and Vdd is required.

GP1 has a 1k resistor, which is effectively in series with the 47k resister in the radio. This should provide about 110uA of drive, again a tiny fraction of the 25mA PIC capability, and about double what is needed by the radio for TX INH. The 1k resistor is there mainly to protect the PIC in-case of an interface cable short – only a little less than 5mA would flow in case of a short.

The circuit has a number of 1nF capacitors. For the ones not associated with the bias tee, I can use some cheap X7R dialectic caps, but little compromise can be made with the 1nF cap in the bias tee. ATCeramics make some good high RF power rated caps for that job, I can use cheaper Ebay caps for the rest.

Finally, the code in the pic, which I will need to write, basically needs to implement the state machine. This code should be pretty straight-forward.

Next step is to design the board, and then it will be time to get some boards made. I have also designed a FT-817 to phone interface for PSK – see post here – and so I will get a batch job done with a few of each of these boards.

73 de Wayne VK3WAM

Part 3 can be found here.