Circuit Notebook 98 - Three methods of measuring frequency in the 10GHz band

10GHz Amateur TV is often radiated by using a self-excited transmitter such a modified satellite LNB, or 'Solfan' Head, as shown in Fig.1. A method of frequency measurement is needed so that you can set and confirm where you are in the band and so maximize your ability to establish a QSO.

Absorption Wavemeter

In my early days of experimenting on 10GHz with a reflex klystron, everything was built in X-band brass or copper waveguide. This included the wavemeter for checking frequency. My wavemeter is shown with its calibration chart in Fig.2. It is similar to that described in the RSGB Microwave Handbook [1].

The wavemeter consists of a tunable resonant cavity and a detector diode and meter. Signals enter at the right hand end of the waveguide and pass to the diode detector at the left hand end. Another section of waveguide is fixed at right angles to the first and signals are coupled into it through a small aperture where the waveguides cross. The far end of this second waveguide is closed off and the near end is fitted with a movable tuning plunger or 'shorting block' which can be moved by the micrometer adjuster so as to vary the resonant frequency of the enclosed cavity.

In use, the incoming signal is rectified by the diode and indicated on the meter. The micrometer is then adjusted until a 'dip' is obtained in the meter reading caused by energy being 'sucked out' by the resonant cavity. The micrometer reading is noted and the frequency is read from a calibration chart. The tuning rate of the micrometer is about 170MHz/mm and the barrel is graduated in 0.01 mm divisions giving 1.7MHz/division.

With care, a frequency at 10GHz can be measured to better than +/- 10MHz, corresponding to an accuracy of +/- 0.1%. Although the wavemeter is bulky, it is simple to operate and gives an indication of signal strength as well as an un-ambiguous measurement of frequency. However it does need initial calibration using a known frequency signal. The best detection of a horizontally polarized signal is provided when the broad face of the waveguide is vertical.

Digital Frequency Counter and Down-converter

This uses a hand-held digital frequency counter (Watson FC-130) connected to a modified satellite TV LNB (Low Noise Block) used as a frequency down-converter, as shown in Fig.3. The LNB contains a frequency converter stage with a local oscillator of (nominally) 9.0GHz. (similar to those supplied by Bob Platts G8OZP) [2]. An incoming signal to the LNB of say 10.340GHz would produce an IF output of 10.340GHz - 9.0GHz = 1.340GHz. This can be measured and displayed by the digital frequency counter. The Watson FC130 is an excellent choice for this purpose as in addition to displaying frequency it also indicates signal strength. The LNB must be supplied with 12-15 volts and this is done using an RF choke to feed the voltage to the LNB coax with a capacitor to block off the DC from entering the frequency counter [3].

The overall accuracy of the system is mainly dependent on the accuracy of the LNB 9GHz local oscillator. The actual frequency can be determined by receiving a known signal and subtracting the indicated frequency. For example, a known frequency of say 10.340GHz might be displayed as 1.334GHz. The local oscillator frequency is given by 10.340GHz - 1.334GHz = 9.006GHz. (This actually applies to my LNB and I have marked it on the plastic cap for future reference, as shown in Fig.3).

This arrangement is very sensitive and can detect and display the frequency of an oscillator circuit several metres away. I use this to monitor the frequency of the DRO (dielectric resonator oscillator) when modifying LNBs for use on 10GHz ATV.

Digital Frequency Counter with Pre-scaler

This arrangement is frequently used to allow a basic low cost frequency counter to measure frequencies in the 70cms and 23cms bands. The incoming signal is passed through a pre-scaler or frequency divider (usually /10 or /100) so that the displayed reading can be interpreted directly.

Pre-scalers for 10GHz have, until recently, been difficult to build and/or expensive to buy. The 12.5GHz pre-scaler XP12-10 by GH Engineering solves this problem for us. It operates up to and beyond 10GHz and conveniently divides by 10 to give a 1GHz output. This is suitable for a UHF digital frequency counter, such as the Watson FC-130 mentioned previously. The GH Engineering pre-scaler is available as a PCB or fitted in a tin box with SMA or other type connectors as shown in Fig.4.

GW8FEY has built a complete 10GHz 'sniffer' frequency counter using the XP12-10 pre-scaler fitted inside a die-cast box with an FC-130 counter fixed to the outside, as shown in Fig. 5. The 10GHz 'sniffer' dipole aerial, resting on the case, is as described by Jim Toon G0FNH in CQ-TV 154 [4]. An internal view, showing the GH Engineering XP 12-10, is given in Fig. 6. The accuracy of frequency measurement depends on the performance of the FC-130 which, although not specified, is probably within a couple of parts-per-million. A CAL adjustment is provided on the counter for a more accurate setting.

My thanks to Barry, GW8FEY, for the loan of his frequency counter.

Figures

Fig.1. GW3JGA/P 10GHz, Llanfairfechan Promenade

Fig.2. Waveguide Absorption Wavemeter for 10GHz

Fig.3. Digital Frequency Counter and LNB Down-converter

Fig.4. GH Engineering XP12-10 Pre-scaler

Fig.5. GW8FEY 10GHz Frequency Counter

Fig.6. GW8FEY Frequency Counter, inside view

References

[1] RSGB Microwave Handbook Vol.2. p10.21 [2] 3cms LNB 9.0GHz Local Osc. Bob Platts, CQ-TV191 p.12 [3] Circuit Notebook No.75, CQ-TV 196, p.17 [4] 10GHz ATV the easy way, Jim Toon G0FNH, CQ-TV 154, p.36. GH Engineering www.ghengineering.co.uk Watson FC-130 (Waters & Stanton) www.wsplc.com