6m MX800 Tranceiver conversion

Rummaging around in the workshop recently I found a cannibalised MX800 transceiver by Spectra Engineering. These appear to be a really well built commercial radio used for repeaters. Lots of fantastic hardware. This was a B series radio covering 70 to 88MHz. We don't have a 4m allocation here in VK so it would need to be modified to 6m to be useful.

Since the receiver board was partially detached from the chassis I pulled it out and brought it down to the house for a closer look. The first thing I did was find a service manual. It showed the front end filter could be padded down to 6m. However, the filter topology was unlike anything I have seen before.

It appears to be  a chain of Pi filters with a series coupling capacitor. I couldn't find any details of the transform used so instead I pulled up the Iowa Hills RF Filter Design software and examined if there was scope to modify this filter to something I could derive values for.

My guess is the inductors on this board were around 60nH with an unloaded Q of 70 based on data I had seen for somewhat similar inductors. While the modification to the layout for a series bandpass filter would be little effort, I quickly established that a series inductor configuration had very little attenuation below 6m so I discarded that approach. No real surprise in hindsight given the small inductors.

Turning to a conventional capacitor coupled bandpass filter I quickly ran up a 3 pole filter with a centre frequency of 52MHz, a bandwidth of 12MHz and standard capacitor values. My initial plan was to run the Local Oscillator on the high side of the received frequency. Attenuation on the image of the local 6m repeater which I would mostly be listening to ( 53.8MHz + 2 x 45MHZ IF) of 143.8MHz was around 35dB. Since there are two of these filters, one before and one after the RF pre-amp, that amounts to 70dB of attenuation.

Which didn't sound like a lot to me if I was transmitting on 2m. My reasonning is 50W at 146.5MHz is 47dBm. Assume this is an ideal transmitter with no phase noise spreading the energy into nearby frequencies. That 47dBm gets attenuated by 35dB before hitting the pre-amp. Again, assume the pre-amp ( an SGA-6489 MMIC) can cope and amplifies this by 20dB before another 35dB of attenuation. Thus the mixer sees a net attenuation of -35dB + 20dB -35dB, or -50dB.

I am guessing but 47dBm from the 2m antenna might result in 0dBm into the 6m feedline. So my 2m transmission is only attenuated to -50dBm at the mixer in the 6m receiver. No big deal if I assume an ideal local oscillator with no phase noise since the mixer is a nice double balanced diode mixer and will probably cope.

But what if the local oscillator has a noise floor that is  60dB below the peak? If the mixer has a loss of 6dB does that mean a 2m signal gets mixed down with 66dB of loss? I don't know but I would be worried that after a lot of work I find every time I transmit on 2m I have to turn off the 6m receiver to avoid the squelch opening.

I've not noticed this issue or de-sensing on my existing 6m radio. I know the existing 6m receiver has much higher Q inductors. It took just minutes to run up a hypothetical filter with a higher Q and a200nH coils and compare it with what I was proposing to build. I concluded the filters I could make using the inductors on the MX800 board would probably suffice.

Comforted by a few minutes work I had a close look at the board. It would appear that by removing all the filter component I could use the inductors and new capacitors with the existing traces.

I am still grappling with the Fractional N synthesize chip. I will continue to look into this but I thought it worthwhile to reinforce my message about filter software with this post. If you're not using the Iowa Hills filter software, a free program, then you should. It took me considerably longer to write this post than it did to model the filters discussed.

Regards
Richard VK6TT

Direct Conversion Receiver - Closing Comments (almost)

I've used this receiver for a while now and in the most part I'm very happy. There are some oddities that a worth mentioning.

No Local Oscillator connected or insufficient level

Without a local oscillator connected or insufficient drive the hex inverter oscillates at around 40MHz. At the same time the receiver audio is very noisy which I find odd. As soon as the local oscillator drive is connected, or increases, the noise suddenly stops when the self oscillation stops. The cause of this has me stumped but in practice it is of no concern.

Dynamic Range

I previously mentioned this and suggested I would use a trimpot to set the audio levels. I made this modification and found it went a long way to easing the adjustment of the audio gain to avoid clipping on strong signals.

However, this did not prove to be the complete panacea for clipping. The problem with setting the gain of the audio chain so you can cope with really strong local signals is you relegate weak signals into the receiver noise.I have concluded this is in due to the way the NE570 operates. I am going to tweak this and report back soon.

In the meantime I hope you find something to build!

Regards
Richard VK6TT


Update: I think I've solved the dynamic range issue. It did turn out that hte strong signal I was trying to listen to was QRO strong. No wonder the NE570 was clipping. I inserted a 36k resistor, though anything from 33k to 47k would probably work, into the signal path connecting to pin 6 on the NE570. See the image below.

 

This allowed the NE570 to cope with this huge QRO signal yet still have enough gain when needed to amplify weak signals.

Direct Conversion Receivers - Remaining challenges

Well I've just about wrapped up talking about this subject. But before you go away thinking everything is covered there are two challenges that you need to be aware of - Dynamic Range and Microphonics.

Dynamic Range

My agc circuit uses a NE570 and has a dynamic range of 60dB. Which I use to cover signals between approximately S5 and S9+30. Signals stronger than S9+30 will distort without some attenuation. And when there are no signals, or low noise levels, the audio gain is running very high. This  creates the potential for instability and is why I spend so long investigating this area. In the end I tailored SMR9 (the 150k feedback resistor shown below) to suit. I ended up with 100k here and pleasing audio resulted for all stations heard. If I was building this again I would make SMR12 a variable 50k trimpot to ease adjustment to cope with the typical stations being received.

Microphonics

This is a recent problem for me. My previous versions were built in a modular form and I never noticed any microphonics, But the sensitivity of my current version is in part due to a low level of noise in the audio circuits. I also built the entire receiver on one board. Both of these factors have contributed to  microphonics becoming apparent.

After replacing all the surface mount ceramic caps in the low level audio stages with tantalums I still had microphonics. It appears these are originating from a 1206 resistor biasing the base of the transistor in the first audio stage. (refer Figure below) I came to this conclusion after watching the output prior to the agc circuit on the cro. Tapping the board anywhere produced a noticeable voltage spike. But slight pressure on this resistor resulted in a 10 or more fold increase in the spike when the board was tapped. Surrounding parts did not react this way.

Figure 1: Highlighted Microphonic Resistor

I have replaced the resistor but there has been no resolution. Looking at the circuit, above, makes this even more curious. Presumably, the microphonic voltage is being injected into the base and appears at the collector. However, I would expect the 10uF tantalum capacitor to bypass any microphonic noise appearing at the base to ground. My thoughts going forward to fix this are:

1. replace the 10uF tantalum,
2. put another capacitor on the emitter of Q2 in case the noise is being injected into the audio signal path through SMR13,
3. mount the resistor on edge, or
4. use a through hole part (defeat!).

I will let you know what worked.

Regards
Richard VK6TT

Footnote: turns out tantalum was bad. I pulled out my ESR meter and it hardly moved the needle. Replacing it cured the microphonics from this resistor. However, with that sorted I found the sensitivity was dreadful. Which sounds odd. After an hour or so of working back through the audio chain towards the mixer it turned out that the 330nF from the two 100 ohm resistors to ground was noisy. Now this is something I had never come across before but must have been due to leakage current from the 2.5V DC across this capacitor. But from before and after tests this capacitor was defintely the culprit so it was binned and another used. Problem solved.

Replacing all the high capitance surface mount ceramics with tantalums and through hole parts certainly created some headaches but all is good now. In the first 20 minutes of tuning across 40m I logged SSB stations from the east coast of Australia, New Zealand and Indonesia here in Perth on the West coast of Australia.

Direct Conversion Receiver - Band pass filter and mixer schematics

Here then is the circuit diagram for the band pass filter and mixer, or product detector. I stress that this receiver was never meant to be a simplistic incarnation, nor was it intended to be complicated. Each iteration was an attempt to improve performance with the least possible effort but sometimes this required more components.

Band Pass Filter and Mixer

Please take note of the comments on the schematic if you decide to make your own circuit board. If you want a commercially made board I may still have some left that reflect these comments.

I tried three trifilar transformers. The first was wound on a really small ferrite bead but I suspect it lacked inductance to work effectively. The second was wound on a larger bead, one that fitted over the top of miniature coax. It worked but I had some intermittent problems with it which I suspect was the result of the enamel insulation being damaged. So I wound a third on a miniature binocular ferrite and it worked a treat. My point is be prepared to try a few variations till you find something that works for you.


Regards
Richard VK6TT