Mixer: JFET active mixer

@5/xx/2012

DC distribution
LO level: 3Vpp , 2Vpp with Gain around 0dB
9V-15V Supply
455Khz IF
LO rejection: 26dB
need more filter to rejection LO in the IF stage: at lest 3 IF transformer as filter

DC distribution

In above box, three blue large inductor located in the corner feed the DC power to 3 BNC. that's is the L1,L2, L3 in Fig.JFET.mixer .

Fig.JFET.mixer

The single JFET mixer is simple but hard to control:

1) bias need carefully test per each JFET to bias the Source to half of Pinch Off voltage.

2) LO driver level is high, here use 2Vpp. MPF102's pinch off voltage is about 3.5V, we should use LO@3.5Vpp per EMRFD's suggestion.

3) good strong signal performance but low gain.

4) strong LO became more powerful, 2Vpp LO get 10mVpp leak at the secondary of the IF transformer. the resonate load of the mixer rejection the LO about -26dB.

What the signal look like from JFET mixer's output? left image get from mixer JFET's Drain, there is many frequncy component, but time domain output is not very help to understand the mixer. the right image below show the signal at secondary of 455khz IF Transformer.

Test condition and result

LO: 2Vpp@1.8440Mhz

RF: 45mVpp@1.3692Mhz

Mixer output: 474.8khz 35mVpp@ secondary of IF T1, with RF

LO leak without RF: 5mVpp@secondary of IF

IF T1 n1/n2: 10:1

LO rejection:

We get 5mVpp LO signal after IF transformer, so we get LO signal at Drain is 10mVpp*10=0.1V. LO rejection 20*Log(2Vpp/0.1Vpp) =26dB

Conversion Gain estimate:

455khz mixer output about: 35mVpp, the convert gain around 20*log(35mVpp/45mvVpp) ~= -2dB. The IFT user by mixer is not very suitable, 10:1 ratio too high, if use 5:1 ratio transformer it get conversion Gain around 5dB.

@5/30/2012

The above testing is some kind-of pitfall, the input of Mixer is too much and easily make the IF stage overloaded.

After eliminate the IF stage self oscillation, here is the mixer output to IF stage's wave form.

Re Test use low power signal: (IF-B0: AGC off) @5/30 2012

input RF: 15mVpp

mixer output: non detectable: too small to the scope to estimate

IF1 output 30mVpp [IF2's signal just same as the above figure]

IF2 output 250mVpp

IF2 gain: 18dB