Sunday, June 23, 2019

Simple 10Ghz frequency counter #2

After previous post on the simple 10Ghz frequency counter # 1, here's another option that will be cheaper than buying a full featured 10 Ghz range frequency counter. Again, different functionalities, this way shown it's just to provide an alternative since Ghz range frequency counters are normally very expensive.
This method involves using an old wave-meter.
Now a days you can find some wave-meters ate reasonable prices, in the range of less than 60Eur like this one from HP that I got recently:

It covers roughly 8 to 12Ghz, it's an HP X532B. Diferent models have different ranges, I wanted 10Ghz so this is the appropriate one.
The wave-meter by it self is not enough to measure a frequency, you will need waveguide transition from coax and a waveguide detector. You can build these two components quit echeaply and they will still work, not lab grade but enough for amateur use.

Bellow the ones I build using copper pcb, not an optimal material but will do:
The waveguide detector part:

Most critical measurement is the distance from the diode to the reflection plate on the back (opposite to the front facing side), what I did was measure 1/4 wave from the end and then adjusted the backplate to biggest signal.

View from the back still not covered:
For the flange mount and bolts placement I just used the template of the wave-meter it self but dimensions are pretty much standard.

For the coax to waveguide transition, same method:

 Signal entrance view:

The antenna can be DC connected to ground or open, I preferred the open option in case I need to provide dc over coax.
Basically I used the pin of the sma connector plus a bit of wire to make the correct wavelength
Again, this is not a calibrated devices but on the overall usage it does not make a difference, as long as we can place a signal at the entrance of the wave meter and detect it on the output we are good to measure frequencies.
The only possible downside is if the waveguide transition block loads the circuit we are trying to measure shifting it's frequency, anyhow the actual frequency after load will be measured. 

I used a Russian D405B microwave mixer as detector since was a cheaper alternative (around 1Eur each plus shipping) to 1N23C and 1N21 that are commonly used in this type of application. The diode arrives in lead sealed container, nice touch, no microwaves will "touch" the diode until is unwrapped.

Bellow a test of the diode with a microamp meter

 At this point if you want to see if your diode works, just cook something on the microwave and while it's running place the diode near the door, should see some deflection on a microamp-meter, you can also use a DMM on milivolt scale.

Dimensions are only critical if you need max optimization.

Now on to measurement:
Keep in mind that this method is not as simple as connecting cable to a regular frequency counter and check the result, you need to observe the output value of the detector and look for very small dips on the output, you need to go really slow in order to find it since it's very sharp and small, it's good if you know the ballpark frequency, otherwise it can be very boring rotating the wavemeter knob back and forth until found since the know is quite de-multiplied.

Same values I measure:

 The dip value above
 The frequency on the dial
and the "normal" value at the signal output.

Bellow the test setup:
 I used my home-brew signal generator for testing and validation

Another view from the top knob:

I build recently a millivoltmeter where I added a blinking led if measured value had over 10% change in the average result so I could rotate the knob faster and just look a the led blink to find the dip. That made my life easier when checking for the dip.

It's the led marked "VAR" in the image bellow

 I will post in the future more information on this millivoltmeter.

Meanwhile, have a nice day.


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