2008-11-23
Heterodyne Sweep Generator
Now days one would probably use DDS to implement such a device, but I wanted to build it the old-fashioned way to work out roughly what I'd face building one that sweeps much higher, to at least 150 MHz... The resulting HF instrument won't be useless itself and will be stand alone requiring no PC to drive it.
The sweep generator is quite simple in design. It mixes a 50 MHz xtal oscillator with a 50-80 MHz VCO then filters out the difference product and amplifies it to a constant level with a levelling loop. While quite straight forward to draw on paper there are a quite a few details that need to be carefully considered to get good spurious-free output from it.
I've gone overboard with blocks in the block diagram, adding lots of pads and buffering amplifiers in an attempt to achieve good clean performance. How many of these will actually be required to make a useful instrument remains to be seen. For now I've started with the key modules, the xtal LO and VCO, filters and the mixer. These 6 modules can be lashed-up in a crude fashion as a proof of concept, which is what I have done (in fact I initially left out the oscillator filters, building the LPF first).
The result you can see in the video, a practically DC to 33 MHz swept source with pretty good flatness even without the levelling loop. My CRO bandwidth is only 20 MHz and its roll-off is much more significant than the level change until 28 MHz. The unshielded prototype is a bit buzzy, the VCO control input having a sensitivity of about 2 MHz/V, but otherwise it is surprisingly stable and well-behaved. The 50 MHz BPF cleaned up some harmonic mixing that was noticeable beyond 25 MHz, as did the VCO LPF. Without a real SA it is hard to test the device further, and largely pointless until it is fleshed out with the extra buffers and pads, and all nicely boxed up.
As a useful device the main thing it is missing at the moment is the ramp generator control system. I intend to build it last though, as it is a relatively straight forward circuit using some op-amps. Presently a 100 k pot is providing tuning. The RF parts present more of a challenge, especially the levelling loop, I've never designed a servo-loop from scratch before but I imagine a primitive 1st order one will be fine?
Development work at the moment is concentrated on making the variable gain amplifier for the levelling loop. I've prototyped a low-distortion BJT/JFET cascode VGA that looks quite promising, but it has virtually no gain when setup to terminate the input at a reasonable return loss. I need to follow this with a feedback amplifier or two capable of delivering at least 10 dBm, preferably more. It would be nice to have a few volts RF into 50 Ohms to play with, say 15-20 dBm.
The directional coupler might be replaced with just a simple moderate-impedance diode level detector in this initial HF instrument. This should offer a stable-enough level for sweeping antennas, filters, etc. The VCO output offers the possibility of using common sweep-circuitry in a HF spectrum analyser, a 50 MHz LC filter and downmixer + xtal filtered IF could be added externally, or even using some of the modules of the sweep generator itself. A log-detector would complete the primitive but useful SA.
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Attachments
| title | type | size |
|---|---|---|
| HF Sweep-Generator Block Diagram Source | application/postscript | 14.497 kbytes |