Re: Observations by W5LUA & AA5AM


I've seen the recommended level for the external reference to be 2 V P-P, which translates to +10 dBm into a 50 ohm load. My OCXO puts out +16 dBm, so I put a 6 dB attenuator on it. I have a 50 ohm resistor soldered across the input from the external reference on the 122 GHz board. I have had no problem locking it so far with that arrangement.

I'll find out this weekend how well it works in the September weekend of the 10 GHz contest, when I actually have someone to work.

73, Zack W9SZ

On Mon, Sep 14, 2020 at 11:55 AM Al Ward <w5lua@...> wrote:

Observations by W5LUA and AA5AM

I successfully modified 2 122 GHz boards for an external10 MHz reference with no trouble. One of my units uses a Wenzel 10 MHz oscillator that by itself puts out +13 dBm. I use an 8 dB attenuator to get the 10 MHz down to about +5 dBm. My second unit is being driven by a Connor Winfield 10 MHz OCXO part number OFC5DJ3Bs which puts out +9.8 dBm. Neither of these rigs had any real locking problems when turned on. They went through the usual cycle of the LED. Blink red, pause and then solid green once you reverse the LED.

Scott AA5AM built and modified 2 122 GHz units to work with a 10 MHz reference. His reference is an Epson TCO-6920N. The output power is approximately +6 dBm. We immediately had startup issues with this 10 MHz reference. Adding a step attenuator had mixed results. Scott experimented with different ways to terminate the reference oscillator and we concluded that the reference oscillator just needs to see 50 ohms and then ac couple the reference to the ADF4351 and the PIC. I went ahead and just soldered a 50 ohm 0805 resistor across the 10 MHz ref input on the 122 GHz board. Scott had placed a short circuit across R47 and C47. I left R45 at 100 ohms used C40 as the dc block. C42 was still in place and obviously L4 was removed. Now the reference sees a nice 50 ohm termination where it did not before. We have not observed any startup problems with this configuration. With the original unit I built with the Wenzel 10 MHz reference, I had it terminated fairly well with the 8 dB attenuator so that explains success there. As I read somewhere else in another post, someone else made a similar observation. I then set up Scott’s 122 GHz unit on one bench and mine on another. My 122 GHz unit with the Connor Winfield reference was connected to a KX3/PX3 running at 144 MHz. Upon looking at the panadapter with the other rig in the beacon mode, I noticed that the spurs were barely down 10 dB. Learning from our experiences with terminating reference oscillators, I put a BNC tee on the output of the reference oscillator and added a 50 ohm termination to the other port of the tee and the passband immediately cleaned up with spurs being 20 dB down or greater. The 2 near-in spurs are +/- 3 kHz from the main carrier. 

I did notice that spurs can increase with a reference level of +13 dBm. +6 dBm seems to be a nice level. I even had Scott’s unit lock with a reference level as low as -2 dBm. Bottom line is make sure your 10 MHz has a nice 50 ohm termination and then adjust reference level for lowest spurious.

de W5LUA & AA5AM 9/14/2020



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