EMRFD Message Archive 12776
Message Date From Subject 12776 2016-05-01 18:24:05 Rod phasing exciter anomalies Hi All,
I built a phasing exciter based on EMRFD and QST artices.
1. Mic preamp from Apr 93 QST "Multimode Exciter for 1-500 MHz"
2. All Pass network from EMRFD fig 9.82, with 10 nf capacitors hand
selected from bag of 30 caps,and 10k resistors hand selected
3. Modulator drive circuit (2n3904 emitter followers) and
SBL-1 mixers from Apr 93 QST "Multimode Exciter for 1-500 MHz";
NOTE, HOWEVER, no MAR-2 amplifier and LPF.
4. LO phase splitter from Jan 93 QST "High Performance Single Signal
Direct Conversion Receiver", figure 6.C
5. 14 MHz +15 dBm into network descirbed in 4. above
6. Mic input 1kHz at 6.2 mv p-p, resulting in 86 mv p-p drive to
SBL-1 IF inputs
1. Carrier suppression -65dBc, ref +15dBm input to splitter,
absolute carrier level -50 dBm
2. desired sideband -24 dBm
3. opposite sideband suppressed to greater than -55 dBc,
ref desired sideband (amazing to me!! Rick, thank you
for the ideas in those pubs.)
The problem: If I increase the Mic input level, the exciter quickly
shows evidence of distortion products. I believe the products are
2nd order, as at the correct frequency for second order products, and
they increase 20 db for every 10 db increase in Mic input.
The All-pass network is not causing the distortion, as seen at the
input of the 2n3904 emitter followers. When I disconnect the emittter
followers from the SBL-1s, and apply a 50 ohm termination in place of
the SBL-1s, it appears the emitter followers are not causing the distortion.
1. is this a reasonable level for distorion products to
start appearing in this circuit topology?
2. Do you believe the modulator drive circuit and fiters/terminations
shown in EMRFD figre 9.83 will clean up the distortion products
and allow higher Mic input drive levels?
Ideas and comments appreciated.