1.1. Circuit

The circuit is pretty much straight from the datasheet and shows a 70MHz low pass filter with 200Ω impedance on the output. The 200Ω resistor (R5) strikes me as strange and was perhaps shown on the datasheet only to represent the output load, and was not intened to be actually used, but nonetheless it’s there on the module.

I connected a 500Ω pot across the output and made measurements of power into various load impedances. Maximum power was with 100Ω which suggests that this is the actual output impedance. I mucked around with a 2:1 impedance ratio transformer to bring it down to 50Ω but concluded that the improvement really wasn’t worth the effort.

1.2. Output Power

Using the oscilloscope, I measured the module’s output level into a 200Ω resistor as the load. 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. I’ll recheck when I get my hands on a newer one.

rudiswiki9 has done some good work on his website here on replacing the onboard filter with external ones, he also notes that the RSET resistor (R6 on the circuit) can be decreased in value to give an output power of closer to 0dBm.

In my case, as I expect to be using these modules quite frequently, I don’t want to be having to modify the modules and building external filters and so decided that it would be better leave the module unmodified and to work with what I’ve got.

2.1. 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 10-turn pot was used to find the value for R1 setting the maximum gain.

2.2. 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. By using the scope’s FFT function as a poor-man’s spectrum analyser, I was able to make two measurements: clean power and maximum power. It’s a little hard to see, but the purple traces in the images below show the difference in the harmonics between the two.

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. Also, the use of the binocular core in the transformer instead of a torroid is maybe suboptimal; I’ll experiment further next time I build one of these amp.

Output Return Loss measured as very good up to 20MHz, and acceptable up to 30MHz, meaning that this amp provides a solid 50Ω source impedance to the device that it is driving.

All in all, I’m very happy with this amp and can see myself using the same design quite extensively in future.

3.1. Prototype

The prototype was built using a similiar technique as for the hycas amp, with the board being the same size so it could sit nicely under the hycas.

As this amp can output 70mW or more at the lower frequencies, I used a 2N2222A in a metal can and superglued to it a hat made from a piece of double sided PCB to act as a makeshift heatsink.

The blue pot was for experimenting with the value of R6, in an attempt to set the maximum gain, but in the end I settled on the value shown in the original design.

The long component leads and big spacing between the transistor and the transformer probably didn’t help with the high frequency response.

3.2. Performance

Unlike the hycas, the gain drops off significantly with increasing frequency.

But nonetheless, it gives a relatively clean output of 8.5dBm at 30MHz and maximum of 18.8dBm (75mW) at 6.5MHz, the input and output return loss are reasonable and the amp should be more than adequate for driving level-7 diode mixers at frequencies up to 30MHz.