Measurements of modules used in rev2 (and some redesigned)

A project log for Microwave synthesizer 4-7GHz

Microwave synthesizer - experiments with microwave black magic

michal777michal777 08/30/2023 at 22:340 Comments

Method of testing

Most of measurements are done using librevna. It has harmonic mixing mode which allows to make some measurements above 6GHz (although be careful with measurements of active devices and fundamental frequency is still there). Calibration was done using PCB test fixtures as shown below (SOLT or TRL possible).


One of the VCOs was tested – 3-3.5GHz. Output level was tested with LibreVNA, previously calibrated using good RF generator. The level is near to 0dBm in the whole frequency range. The results look ugly because 1. LibreVNA is not spectrum analyzer and shows the unwanted tones, 2. no shielding, 3. no closed loop PLL, just VCO alone so the low frequency noise is not filtered by PLL.

Example measurement: VCO 3-3.5GHz, at 3GHz, 50R added in series, measured value with added 10dB attenuator - power -1.5dBmThe second test of VCO is S11 at output. I simply measured VCO using VNA while it oscillates but I’m not sure if it’s good method. It seems that the VCO output has low impedance (probably it makes sense – it’s OC circuit) so adding 50R in series at the output decreases S11.

Example measurement: VCO 3-3.5GHz, at 3.5GHz, 50R added in series, S11 away from oscillation frequency - about -10dB

VCO selection switch

Not tested but expected to be not worse than filters switch

Variable attenuator

Typical application of BAP64Q 4-diode attenuator – 2.7dB insertion loss, 23dB attenuation at 3.5GHz.

Full off:Full on:

2-3.5GHz amplifier

Initially there was a weird shape of gain characteristic.

It disappeared after removing some decoupling capacitors – instead of 2.2pF parallel with 47pF, only 2.2pF was left. It looks like some resonance was caused by the 47pF capacitors parasitic inductance (C707 and C703 removed).

Simulation for comparison:


It’s hard to test because I’ve no proper equipment for the task. I tried testing is in connection with amplifier to drive it with reasonable level and output filter to have only one tone – then the signal is measured with a HP 33330B detector. Power of the signal with amplifier is compared with power of the signal after doubler and filter.

Loss of the doubler is probably about 10dB (13-16dB including with filter)

Also I did simulation in spice which confirms about 10dB loss and confirms that doubling works best with not too high input power, less than 10dBm.


Filters were measured and the results were ok, loss better than 5dB on FR4, only center frequencies should be slightly increased.

Switches for filters

Only one diode tested. The switch was redesigned. Two PIN diodes in series provides better attenuation when off and have not bad insertion loss when on. Larger pieces of copper between the diodes are to partially compensate diodes inductances by adding some capacitance (works in simulation, not sure if also in real world).


Output amplifier

Previous version of 4-7GHz output amplifier was very similar to 2-3.5GHz amplifier but didn’t work as well and I couldn’t determine why it differs so much from simulation. I failed to find a way to tune it so that gain doesn't drop that bad at 7GHz. Insufficient output level in was the major issue in rev2.

New amplifier was designed, measured and tuned.

Gain of 3 amplifiers connected (note: 2x 10dB attenuator installed – real gain is 22-27dB).

Output filter

Low pass output filter was removed because it probably didn’t work well (acc. my inaccurate measurements, too much loss at 7GHz and poor attenuation above).


Improvements of PLL filter was important for phase noise. Initially, with both PLLs performing poorly I couldn’t get any results with RBW set to 1kHz.

Dedicated software was used for the design, together with testing, trails errors.

With both filters for both PLLs (1st and 2nd LO) RBW 1kHz makes more sense. The filter removes a low of low frequency noise (which is large, especially in the prototype without any shielding)