Amplifilter

g0mjw · Post by **g0mjw** » Fri Oct 24, 2014 5:46 pm

I am considering how to boost the 0 dBm or so of OFDM from a DATV-Express to a few watts. The problem I have is the amplifier needs to be extremely linear. The difficult region is for bands up to 23cms. At 2.3 GHz its easy as there are plenty of ex 3G amplifiers around, typically taking in 1mW and developing 10-20W linearly, or 100W at 1dB compression. Ditto 3.4GHz. Above there we are into QPSK only and linearity is a bit less of an issue because we are less constrained for bandwidth but again, there are surplus amplifiers that only need a few mW of drive.

For 2m/70cm/23cm there seems to be less of the high gain ultra linear surplus kit about. The need I have is to get to around 30dB of gain to 0.5-1W which is then sufficient to drive a commercial or home made amplifier to 10-20% of its rated output where, if it is a decent design it will be linear enough for DATV. I built a mini-kits +29dBm amplifier for 23cm. It will develop 1W but it only produces about 200mW linearly and its not 100% stable across all input levels even into a high quality load. This is not quite enough, I could do with more like 2W on 23cm. For the lower bands I have a collection of amplifier modules that when combined will get me up to about 100mW sufficiently linearly. At a push that's just about enough, but another few dB would be useful. Its all a bit of a kludge though requiring several modules in series.

Amplifiers need harmonic filtering. I have cavity filters for the higher bands and helical ones for VHF but those are not appropriate because they don't suppress harmonics. I am looking for low pass filters able to handle high power. I have successfully built these with coils and capacitors for for up to 70cm but lumped components are becoming difficult for 23cms and up. However, LDMOS amplifiers like those based on the MRF287 amplifiers are pretty common now and certainly need harmonic filtering. I know everyone with such an amplifier must be using a harmonic filter, but it is a mystery where they get them as I have not seen any designs.

Mike