AD9850 DATASHEET PDF

Anyway, I finally decided that it was time to bite the bullet, make some measurements, and come up with an amplifier that would provide the basis for a worthwhile HF signal generator. AD Module 1. You can see that it declines rapidly with increasing frequency- so much so, that it seems that something is not right. It was mentioned somewhere that perhaps some of the modules have the wrong value components in the LPF. As the graph shows, the cutoff frequency seems to be closer to 8MHz than to the 70MHz required. The modules I tested were 18 months or so old, so perhaps this problem has been fixed.

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Anyway, I finally decided that it was time to bite the bullet, make some measurements, and come up with an amplifier that would provide the basis for a worthwhile HF signal generator. AD Module 1. You can see that it declines rapidly with increasing frequency- so much so, that it seems that something is not right. It was mentioned somewhere that perhaps some of the modules have the wrong value components in the LPF.

As the graph shows, the cutoff frequency seems to be closer to 8MHz than to the 70MHz required. The modules I tested were 18 months or so old, so perhaps this problem has been fixed. Thanks to ka7exm for his article explaining these amps, and to Vasily for his extensive practical applications on his now defunct PopQRP site.

The advantages of the cascode arrangement are that it gives wide bandwidth due to minimisation of the Miller Effect w2aew has a great YouTube video explaing this here , and secondly that the gain can be easily varied by the voltage at the base of the BJT.

Furthermore, by using the hybrid, the input impedance can be very high, meaning little load on the preceding stage. All of these things make the hycas ideal for this application.

RV1 varies the gain and the Schottky diode D1 provides supply polarity protection; even with polarised power connections and multiple checks, I still need it! Cheap protection. R2 sets the input impedance, 10k seemed like a reasonable value to provide high input impedance and yet to provide enough current to not be effected by the gate capacitance of Q1.

Prototype The protoype was made on a piece of double sided PCB, using, appropiately enough, a hybrid Dremel-etching, Manhattan and Ugly construction method.

Single sided board was used for the front panel and for the rest of the case. To ensure a good connection between the top and bottom ground planes, plenty of wire vias were used, especially near to component leads connected to the top ground plane. Also, the the front and rear panels were soldered to top and bottom ground planes.

The panel-mount SMA connectors are handy in this type of constuction as they can be soldered top and bottom, giving them quite some strength. Once the box is finished I glue around the edges to provide for some extra stress relief. The blue turn pot was used to find the value for R1 setting the maximum gain.

Performance I measured the output power and gain at various frequencies, but noted that at the lower frequencies and with high gain, the waveform would become quite distorted. As the graph below shows, output is at 8dBm or greater up to 20MHz and gain is relatively flat to 30MHz. I suspect that the dip in gain at the lower frequencies may be due to the drive level from the DDS being too high, and hence causing gain compression.

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