Radio Board. Fix mixer LO low signal issue

[chrisdebian]

Describe the bug:

The LO input is 7dBm (1.4Vpp) with the ring mixer disconnected. This must be over the frequency range 8 to 155MHz.

Steps to reproduce:

Disconnect the mixer LO pin. Measure with Oscilloscope (Vpp) or Spectrum Analyser (dBm). NOTE: Be aware that the scope has a highish input impedance and the SA a 50R input meaning that doing measurements can load the attenuator cursing the results. Also in my setup I have discovered my 100MHz scope probes are very low quality and once the signal is over 40MHz they attenuate a lor making them pretty useless for this measurement!

Any thoughts on a solution to the problem?:

Firstly, Getting an accurate measurement method is a must then ensuring the 2n3904 buffer/amplifier is suitable. It is I believe going to be ok for HF but for all frequencies I am doubting so a new design might be needed. Assistance from a good RF engineer is needed.

[HamPiRadio]

Progress by Michael:
Measurement method: This of course has to be established first. The Spectrum Analyzer presents a 50R load to the circuit. To get an accurate measurement a quality probe is needed, and what works is a crimped SMA coax direct onto the circuit. This measure accurately using the tracking generator at -20dBm , and also the Signal Generator set to the same.
Requirement clarification: The post amplifier is driven at a high level into a Pi Attenuator that then feeds directly into the Double Balanced Mixer. I have changes the two 2N3904 devices to 2N2222a transistors (in parallel but with 12R emitter resistors) as they are higher current. The reason for this is to reduce any noise. (Refer to the original OMEGA transceiver write-up for this).
Results: As of last night the 2 x 2N2222 devices are heating up! Next to check the drive level again and the output loading...

[chrisdebian]

Michael, please could you add a short status update, even if nothing has changed.

Many thanks,

Chris.

[HamPiRadio]

I'm happy to. There has not been much progress recently apart from a) The work with yourself. b) Getting hampiradio.com up and running again. It is now secure. c) the radio-board-VFO thread on Discord. I have been doing a bit on this and the amp is now stable. As per the Discord thread there are a few components that need values to be decided on and a peer review or someone else to breadboard this would be great. d) the other issue (Rf Feedback) No progress as the VFO issue above MUST be sorted first. Thread on this is on Discord.
Software is stable and am very pleased with this. I will up load the UF2 soon. Next CPU board is designed and laid out. Needs BOM and JLCPCB parts need adding and production. Front panel is also ready. The old and now removed Function rotary control are gone. Have added 3 SMD leds and removed the large hand mounted ones. They now fit on the Keypad PCB and I have found nice lenses that fit in front of the leds in the front panel.
PA and BPF boards are working fine, but I am / will be working on an iteration to manufacture them with as many smd parts as possible. Just needs time.

[chrisdebian]

Hi, Michael.

Thanks for the update. On Discord, you shared some diagrams, would you be able to upload the relevant ones here? perhaps do the same for the other bug (no. 2)?

Cheers,

Chris.

[HamPiRadio]

Oops, most of that update was not limited to the VFO issue! Apologies.

[chrisdebian]

Oops, most of that update was not limited to the VFO issue! Apologies.

Easily done, Michael. No worries.

Chris

[HamPiRadio]

These are a copy of the pictures on Discord for this VFO thread. M

[chrisdebian]

Thanks, Michael; that'll help the peer review.

Chris

[HamPiRadio]

Morning Chris,  I uploaded the radio schematic. If you have kicad loaded you should see it, but there are some custom symbols I need to upload, so some of it won't look good. Can you check and email me corrected working on the bottom right please, about usage?Thank you.   Michael Sent from my Galaxy -------- Original message --------From: chris_debian ***@***.***> Date: 28/04/24 11:02 (GMT+12:00) To: HamPiRadio/HamPiRadio ***@***.***> Cc: Michael Wise ***@***.***>, Assign ***@***.***> Subject: Re: [HamPiRadio/HamPiRadio] Radio Board. Fix mixer LO low signal issue (Issue #1) Thanks, Michael; that'll help the peer review. Chris —Reply to this email directly, view it on GitHub, or unsubscribe.You are receiving this because you were assigned.Message ID: ***@***.***>

[chrisdebian]

Hi, Michael.

I can install Kicad, but wasn't sure what you meant by "email me corrected working on the bottom right please"; could you clarify?

Thanks,

Chris

[HamPiRadio]

Hi. The transistor is now a 2N2222a. The DBM device can either be a SBL-1 or an ADE-... device. That works well already. Thanks. MSent from my Galaxy -------- Original message --------From: chris_debian ***@***.***> Date: 29/04/24 00:39 (GMT+12:00) To: HamPiRadio/HamPiRadio ***@***.***> Cc: Michael Wise ***@***.***>, Assign ***@***.***> Subject: Re: [HamPiRadio/HamPiRadio] Radio Board. Fix mixer LO low signal issue (Issue #1) For consideration, and to be attributed if the originator would like: "Consider replacing the 2N3904 and 2N2222A and use the BFR106. Check out the ADE-6 for a DBM" Thanks, P. Chris —Reply to this email directly, view it on GitHub, or unsubscribe.You are receiving this because you were assigned.Message ID: ***@***.***>

[HamPiRadio]

The issue isn't the Si5351, 2N2222a or the Double Balanced Mixer. It is getting the AMP levels correct and the attenuation matching as per my comments on Discord.