[Argus25]

My take on this circuit is that it is an attempt at a current source driver. Before I'l explain it I could mention that there are two basic types of problems that cause non linearity or distortion in AM detectors which use diodes. (These problems can be cured using op amp precision detectors, but that is outside the scope of this response)

Firstly there is the problem (for a detector with a positive output polarity) of negative peak clipping that always occurs when the modulation reaches some frequency threshold. It occurs because of the RC time constant that the filter the detector feeds, has an exponential discharge profile. The frequency at the point this distortion starts to appear is approximately 0.375/RC when the modulation is around 50%. It's a lower frequency when the modulation is higher. Basically the detector diode gets reverse biased due to the charge stored on the filter capacitor on the negative modulation peaks and it's not able to track the modulation envelope.

Secondly , the distortion caused with low input level to the diode detector that affects all modulation frequencies when the signal level to the detector falls too low. The detector, for low level signals has an approximate square law property, or close, rather than a linear transfer function and also for very low level signals fails to detect at all.

The cure for this problem is to drive the detector with a current source rather than a voltage source. This is elegantly explained by Horowitz & Hill, with an example circuit which converts the incoming modulated RF voltage to a current source to drive the detector diodes.

This works because in the case of a circuit attempting to provide a current drive, the voltage rises as high as it needs to develop conduction in the diode even with an extremely small RF input signal voltage. But there are other ways to do this, one way is to source the current from an inductor or transformer when the voltage has been stepped up prior to the diode, this apparrent mismatch achieves the same effect.

In the case of the circuit provided by the OP, it could be regarded as a circuit that converts the modulated RF into a plate current. From the voltage perspective the plate current is converted to a voltage by the 10k anode resistance, and the higher voltage swing at the plate ensures that the detector diode current remains high even for very low level signals out of the RF transformer. In addition this configuration puts little load on the IF transformer's secondary, helping to maintain the gain and selectivity. So I would expect this type of circuit to be better than most in detecting low level signals with little distortion.