Choosing a mixer for HF transceiver

Hello guys,
I'm trying to design an analog portable single side band receiver (ham radio transceiver in future).

I am looking for a good matching components.
Initially I started with SA612 mixers but I was disappointed by their noise level.
So I tried AD831 which had almost no gain while dissipating tonnes of heat. I understood I was foolish underestimating their power consumption and made many mistakes.

I have drawn this block diagram:

It uses Si5351 as VFO. First mixer VFO is set above receive frequency. RF BPF is up to 2MHz wide. IF 8MHz BPF is 3 KHz wide. LO can move up & down to be able to receive USB or LSB.

Now I am trying to build something which will be power efficient, compact.
So I picked LT5526 as double balanced mixers.
But I am looking at LT5560 and LTC5562.
LTC5562 seems having better characteristics and claims to have ESD protecting which sounds important for HF applications having an external antenna. The only caveat it has some note about external matching for lower frequencies which is my case.

For IF amplifier I picked AD8338. It has attracted me by presence of AGC and 80dB of dynamic range. But I struggle a little bit with proper configuration for this chip. I had to manually set AGC setpoint. Also I think I made poor decisions regarding my PCB design, I guess it starts self excitation. Many people recommend 4 layers board but it is an overkill for ham radio. I think it is doable with 2 layers. Also I was thinking to tap to AGC loop and use it for S-meter.

Next, I am thinking about some LNA after RF bandpass filters but unsure about it. In different designs I see LNAs and attenuators there. If signal will be stong it will overload LNA and small signal might be lost. From the other side, ham radio signals are usually very weak unlike broadcast radio stations so it worth to have it.

For audio amplifier I used SSM2211 and LM358 as preamp/filter. It is the only one which has option to have single ended output which is one my requirements.

TBH, I am not RF engineer and radio is my hobby so I am very welcome to any advice.

typo fix
[edited by: zoonman at 4:21 AM (GMT 0) on 21 Jan 2020]
  • Hello,

    I think nice design, well described. Did you check your frequency plan? I am not sure, but if I understand the solution correctly, 1st IF (8MHz) is in the input RF band. Or is there a gap in the RF received band? If no, you should revise the frequency plan and use e.g. 1st IF above the RF band. You have to decide about the noise figure. Usually the noise figure is requested ~3dB (or less) for the VHF, UHF bands but I don't have experience with the HF band, I assume the noise figure can be higher because of higher noise background. To reach better NF, LNA shall be placed in front of mixer of course at the expense of linearity. Define what you want to reach (NF, IIP3) and calculated level diagram gives you answers whether use or not to use gain blocks somewhere in the RF chain. 

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    •  Analog Employees 
    on Jan 21, 2020 7:36 PM 10 months ago in reply to viktor_vojta

    I read SA612 datasheet and noticed that it has just 5dB noise figure which is smaller than all ADI mixers. Now I am very confused.

Reply Children
  • Ok, understand, so 1st IF is positioned in the unused RF band-gap. 

    I think NF is more important than IIP3.

    I can not agree with you. Both parameters are important and specially nowadays the IIP3 is probably more important than NF. Everywhere are different sources of interferers which increase the noise background. I want to say if you design RX with extremely low NF, the receiver will receive and amplify useless noise from different SMPS, industrial instruments, cabling, which generate EMI spectrum mainly in the HF band. As you know, the NF is above all defined by RX front-end and IIP3 by 2nd IF. 

    4 layers board but it is an overkill for ham radio

    From my experience, the price difference is not significant between 2L and 4L boards. Especially if you don't have special requirement and you use any pool-service.

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    •  Analog Employees 
    on Jan 22, 2020 3:31 AM 10 months ago in reply to viktor_vojta

    I was looking at LNAs and I cannot find anything in a reasonable price range. Probably something like BGA2866 will suit my needs.

    Regarding 4 layers PCB here are the quotes:

    4 layers

    Almost $60.

    2 layers$27.

    I think 2x price difference is huge. Even $27 for a hobby project is a bit expensive.

    I will give a shot for LT5526 as a mixer. Also I recently learned about critical path so if my tracks are short enough impedance mismatch will be negligible.

    Also I thought about RX/TX switch and I think HMC544A should get the job done. 

  • Yes, I was wrong with the PCB price assumption. Usually I use local PCB manufacturer and the prices are (after currency conversion)...2L:39$, 4L: 76$. So you are right, sorry.

    LNA seems to a good choice, your design is really well price optimized. On my previous project I used gain blocks from OnSemi, these are also very cheap. Supply goes thru coil (e.g. for SMA3103), it can be benefit for the LNA linearity. But better way is to calculate level diagram first and search the LNA not just for price but also with known gain, NF and IIP3.

    I don't recommend you to use switch HMC544A. It is a GaAs switch, so you can expect strong degradation in linearity below ~10MHz. For the RX it is not an issue, but in the TX mode the harmonics distortion will be high. 

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    •  Analog Employees 
    on Jan 23, 2020 5:01 AM 10 months ago in reply to viktor_vojta

    OK, lets do some math regarding overloading LNA.
    Here, in Boston we have 25kW AM broadcast station working at 1.33 MHz, I think it is the most powerful one here. This is 74dBm. Lets assume that we have perfect antennas on both ends and no losses in transmission lines. The station located approximately 5 km from me which means -48dB free space path signal loss. I used some online calculator to get the estimates.
    Normally, Butterworth 3rd order BPF will have loss around 6dB in band pass region and around 40 dB in rejection region at 1.3 MHz.
    So, what we have: 74 - 48 - 40 = -14 dBm.
    BGA2866 has -18 dBm input power at 1dB gain compression. Given real antennas, losses in cables, connectors, switches we easily loose these 4 dB and fit into desired LNA's Pi(1dB).
    Worst case, we can bypass this LNA and +10dBm for LT5526 is totally fine. That translates into about 100W for ham radio transceiver inside pass band with bypassed LNA located 5 km from receiver. Again, no losses.
    If we want to have higher dynamics we have to pick LTC5562 which has +15dBm Pi. That's teasing me now :-) Maybe I should use it.
    The only caveat - I am unsure how to properly design injection of audio signal for transmission path and acquire audio from second mixer.

    What would you recommend instead of HMC544? Relay? My plan was to get rid of relays whenever possible and use latching ones for LPF in transmitter. I know, I still have to switch antennas using relay.

    Thank you for your help, Victor! This is incredibly valuable for me.

  • Thank you for excellent explanation, it is very interesting for me. Nevertheless use fist LNA very near P1dB is risky, it will intermodulate for sure. You can not assume, that any amplifier is perfectly linear up to P1dB and then is hard break in the characteristic. P1dB is more important for TX design, for you is more important IIP3 on the RX.

    Yes, I would recommend RF-relay. I know, it is an issue for life time, but I would go this way. I have seen nice and cheap relays from COTO, e.g. 9814-05-00TR or 9814-03-00.

    (Don't thank, my English must be horrible).