SDRZero G2
SDR Transceiver
2/feb/2007
updated 14/May/2007
The goal is an exciter with
very good IMD performance, above average. The intended power output is 5
Watts, good to worldwide QSO on 20m and enough to drive a 100W block. A pair of
IRF510 is sufficient, availability and cost are attractive - RF specific
transistors are expensive and not so easily found.
According to some amplifiers based on IRF510, a gain of around 14 dB can be achieved. For 5W output we would need 0.2W (23dBm). Our exciter driver must be able to put out this power level.
EXCITER
As a first exercise, a driver
stage using two BGF591 transistors in push-pull was designed and simulated.
This transistor choice was a suggestion from Ulrich Rohde, KA2WEU in the HPSDR
list. It is a very interesting part, intended for CATV linear use, with 2W max
dissipation and Ft = 7 GHz.
The push-pull topology was
chosen in order to attenuate the even harmonics, this will make output
filtering more easily attainable.
The circuit is fed with
12V. The quiescent current per transistor was chosen to be 60 mA. Thermal
considerations will dictate the standing current. This will be easier with a
breadboarded working circuit than just going through the numbers from the
datasheet and doing the math.
Series and shunt feedback
are employed. A good treatment of this matter can be found in Wes Hayward, W7ZOI
book "Introduction to Radio Frequency Design". For general theory and
design techniques a good recomendation is "RF/Microwave Circuit Design for
Wireless Applications" by Ulrich Rohde and David Newkirk.
S-parameters, 0~2 GHz
S-parameters, 0~100 MHz
2-tone 14/14.02 MHz span
0-100 MHz
2-tone 14/14.02 MHz span
13.9~14.15 MHz
Stability factor K. From 400 to 900 MHz stability is conditional, the amplifier needs some damping or selective feedback
Two-tone power input sweep vs power output and 3rd order IMD product
Each transistor colector current vs input power sweep. The amplifier operates under class-A conditions up to 2 dBm input power, then it enters class-AB region.
Noise; This amplifier could be a very robust HF receiver pre-amplifier with NF = 4.8 dB and output IP3 43 dBm.
Part 2 - Fixing the stability, simplifying and improving some numbers. Unconditional stability is obtained, OIP3 is raised to more than 50 dBm, our driver stage is taking shape.
MOSFET PWR AMP