23cm Swept Oscillator

If you read my earlier post, here, I covered the background to a 23cm VCO I built. While this vco is likely to be used in a transceiver one day, it also became part of a useful piece of testgear - the swept oscillator. While there are other ways to align filters, using a swept oscillator rates highly on the bang for buck scale.

 Principle behind use

Principle behind use

The basis concept behind the use of the swept oscillator is shown above. The frequency output of the swept oscillator is connected to a radio or a stand alone filter. Somewhere from the radio a response signal, perhaps the AGC line, is picked up and fed to the Y axis of a CRO. The CRO doesn't have to be fast, since the signal has been detected and the rise time of the detected signal is slow.

The sweep speed has to be fast enough to create the persistence of vision of the CRO's trace, but not so fast that the detector has not had time to react. A sweep of 25 to 40 times per second works for me.

My implementation

At 2m and 70cm I took the expedient path and hacked a commercially made oscillator. But at 23cm I had no alternative but to build a swept oscillator. A picture of my swept oscillator, working but not quite finished, is below. The maximum sweep is from 1230MHZ to 1406MHz.  You will notice the piece of copper foil I soldered onto the middle stripline to lower the frequency range it was covering.

The three controls are sweep speed, sweep width and centre frequency. The bnc connector connects via a patch cable to the X axis of the CRO. The N connector is where the RF frequency is sourced. I use an external attenuator of at least 15dB to ensure the filter or radio thinks it is connected to a 50 ohm source.

VCO
I covered this previously, here, so I'll just repeat the schematic.

Ramp Generator
I have used this ramp generator many times. It was based on something I saw at Hans Summer's website but with a small correction. It took me a long time to work out why, at the highest point of the waveform, the voltage did not reverse but instead momentarily spiked to the supply rail. It was caused by something called phase reversal and is the result of the op amp driving a capacitive load.

The fix was a small value of resistance, 330 ohms, in series with the output of the op amp to isolate the capacitance.Works a treat now.

Sampling a response

There was a time when sampling the rf coming out of a filter with a diode detector was the norm and it wasn't possible to worry about the response being linear or logarithmic.

Today there are logarithmic detectors that work on low level signals. The AD8307 is useful up to, and including, 70cm. At 23cm I use the AD8314. I will write these projects up soon because my implementation of these logarithmic detectors has enhancements I have not seen anywhere else.

If I am sweeping a stand-alone filter then the output of the filter, with perhaps 10dB of attenuation to improve the match to 50 ohms, is connected to the AD8314 chip. The output drives the Y axis of the CRO.

Where I am trying to sweep a filter already in a radio then I modify my approach. I haven't done this at 23cm yet because I don't have a radio to align. But I have aligned numerous 70cm and 2m radio's with the following approach and I expect it to work at 23cm. I have a dud bnc patch cable with the dud connector cut off. This open end of the coax becomes my sniffer probe. I simply probe around the radio after the filter until I see a response on the CRO. Then I align the filter. I am not making a direct connection. The AD8307 or AD8314 are sensitive enough that capacitive coupling is all that is needed.

PCB
I have a pdf of the artwork if you would like to build one. I don't know how to put this into the blog so a jpeg image is shown below to whet you're appetite. Note that it is not actual size!