Plans for a scalar network analyzer

The signal source project is developing into a scalar network analyzer, and to measure transmission and reflection parameters, a detector is required. After a few diode diode detector designs, which had a low dynamic range in terms of power, a difficultly wide dynamic range in terms of output voltage to be digitized and an unpleasantly nonlinear response, I decided to go heterodyne. Mixing the sweeped signal to a low frequency allows the use of a logarithmic amplifier as a detector, giving a high dynamic range and a log-linear output with easy analog processing. However, at this point the system complexity takes quite a step.

Above is a rough block diagram of the current plan, having already gone through numerous revisions. A number of components were bought for the previous revisions, and then left unnecessary after updates. The microwave relays, couplers, mixers and the power splitter are from eBay. At this point it seems all the finished boards will end up getting redone. Some kind of plans for this have been brewing for 15 years or so, but for the moment the system has reached some level of feasibility and system level maturity. Perhaps something will actually get finished some day...

Though noisy, it makes sense to restrict adjustability to the YIG synthesizer. In order to get a low and constant intermediate frequency for the log amp, the sweeped frequency needs to be mixed with a suitable signal. A 315MHz IF was chosen for the log amp simply because filters for that frequency existed in my component stores. Filtering 315MHz from a signal spanning many GHz would be difficult, so the 4-18GHz YIG output signal is mixed with a constant 3.65GHz signal. A couple of microwave relays select a 2.7GHz low pass filter or a YIG filter with a passband adjustable between 2 and 22GHz, with a bandwidth of around 60MHz. Stretching the output of the YIG synthesizer a bit by replacing the original driver with a self made design, the YIG will go down to 3GHz and up to perhaps 20GHz if needed. This will allow the system to sweep the transmission response of the device under test from 0 to 22GHz. A wideband coupler in series with the output connector also enables the measuring of reflection between 2 and 18GHz.

On the receiver side the YIG synthesizer output is mixed with the signal coming from the DUT (or the reflection path coupler), resulting in an IF of 3.65GHz, which is filtered and then converted to the final 315MHz IF. This requires a separate synthesizer with a frequency of 3.335GHz.

The YIG synthesizers output power varies between +11 and +18dBm, and is further attenuated by the power splitter. With a direct measurement, this variation would need to be corrected by a wideband adjustable attenuator and a feedback loop with a detector and a calibration system. However, in the heterodyne design the variations are reduced quite a bit as the YIG synthesizer output is used only as mixer LO signal, and the transmit side mixer RF input frequency and thus also signal level stay constant. The LO level variation affects the output level very little. The mixer and the filters will have somewhat frequency dependent losses, but I don't yet know whether they are large enough to be bothered with, the total transmission response will be calibrated on the receive side anyway. If some level calibration is required, it is simple to implement by adjusting only the level of the constant frequency synthesizer output feeding the transmit side mixer RF port.