Journey to a small stand alone CW signal synthesizer

[Da_Stier]

Since I do a lot of experiments, where I need many different frequencies, I was in need of a small standalone source to generate these signals.
I do own two nice RF signal generators, however they are huge, very expensive and quite overkill if I just need a single tone.
So I came across a cheap eval board for the ADF4351 synthesizer IC from Analog Devices on ebay.



At the time they cost around 40€ shipped which was an ok deal at the time.
They can generate anywhere between 35MHz and 4.4GHz. The amplitude is mainly fixed and at around -6dBm.

After playing a bit with the module, I had a list of pros and cons for it:

+ relatively cheap
+ nice frequency range
+ small

- the onboard reference is quite bad
- you have to change a solder jumper to use the external clock input
- not standalone since it has no controller
- no shielding

So I decided I will just run my own PCB for the IC and tackle all the prementioned points.
This was the first board I came up with.







While it did work, I still was not really satisfied with its performance.
Since the PCB is just normal FR4, the amplitude responce at higher frequencies was terrible (attenuating up to 14dB).
The RF switch that I used for switching over the clock reference only had about 30dB of isolation in the offstate which wasn't enough if you connected an external source but wanted to use the internal one.

And the biggest point... it was expensive.... one fully populated board had a BOM cost of around 65€ which I found pretty high.

After looking on ebay again, I saw, that the original eval boards came down in price quite significantly, selling for only 15€ now.
So I decided I would try something a bit different for the second version of the board.











I decided to use the eval board as an drop in module like I use the arduino boards.
This works really well and basically solves all the problems of V1.

The last step was some mechanical design.
I milled some aluminium shielding cans and printed some plastic lids for the bottom.





So this is the final module. It costs me around 25€ to build and can output between 35MHz and 4.4GHz.
It is controlled either via USB or UART, both connected over the DB9 connector on the front.
By using the external clock, I can lock them together on the same reference, which comes in quite handy sometimes.

I hope some of you find this small journey of interest.

Greetings,
Michael