DFS Issue 2 VHF Diplexer Information

G4HUP DFS Issue 2 VHF Diplexer Information
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Adding a diplexer to the Iss2 DFS PCB

With the introduction of the Issue 2 PCB, new possibilities to create DFS solutions have been provided, because of some new features. When using solutions with widely spaced input frequencies to the VHF mixer (M601), it has been proven to be advantageous to use a diplexer after the mixer.

The Diplexer Circuit

The circuit shown includes the VHF bandpass filter (C1-5 and L3,4) from the existing DFS circuit diagrams (C601-5 and L601 and 602). The diplexer iteslf consists of two series tuned circuits - L1, C7 are tuned to the wanted frequency, and pass it through to the BPF. L2 and C6 form the other circuit, and ar tuned to the image frequency, offering it a low impedance path to the termination, R2. In theory this should be a 50ohm resistor (47R or 56R is OK). In practice I have found no advantage in providing it, and the notch in the slope appears to be deeper if it is 0R. R1 is a low value input resistor, and the mixer output is connected to TP1.

The combined response of the diplexer and BPF is shown in the Ansoft simulation above. There is a clear notch of almost 20dB - in this case on the low frequency side of the BPF slope. Because it is tunable, this diplexer circuit will suffice with the component values shown, for all DFS implementations that use a 90MHz VHF LO and have outputs in the 100 to 120MHz range. Recalculation will be necessary for other LO frequencies, and for cases where the LSF is used.

Preparing the PCB

Some modification to the PCB is necessary to accomodate the new components, and two existing components must be re-located. This work is much easier to do before the PCB is assembled into the box.

Locate M601, the VHF mixer, and the output track towards the BPF. With a scalpel, remove all the solder resist along this track, on both sides of the capacitor.
C601 and C602 must be re-located adjacent to the pin of the inductor in the BPF. Cut across the track close to the pin, so that C601 can bridge the gap.
Remove the resist from a part of the ground plane, so that C602 can be placed between the end of C601 and ground - again, try to make it as compact as possible.
Make a cut across the track close to the mixer, for the new input resistor, R1. L1 will fit in the position previously occupied by C601.
There is a portion of ground plane close to the track you have been working on - remove solder resist from that - there are no vias in the area you need to clear. This will become the shunt circuit to ground.
L2 will fit from the junction of L1 to the new pad. C6 must bridge across to ground (ignoring R2)
the original C602 position is not used.

The view above shows the modification as applied to the 108.667MHz DFS. Note that the shunt path L and C are the other way round to the recommended locations in the bullet points above.

What is a diplexer and why do I need it?

The output of a double balanced mixer contains the sum and difference of the input frequencies, and a suppressed copy of the LO signal (usually around 30dB down). The sum and difference frequencies are equal in level at the mixer output, and we select the one we want with a filter. This filter will present a nominal 50ohm impedance to the mixer output - but only at the frequency of the wanted signal. At other frequencies, the impedance will not be 50ohms.

However, for correct operation, the double balanced mixer needs to see a termination which is 50ohms for all the output frequencies. Without this proper handling, the presence of these unterminated signal components can cause problems in later stages, and are visible as spurious products. When correctly terminated, their levels are minimised. See the comparisons below.


Output spectrum of DFS without diplexer	Output Spectrum with diplexer installed

Page created 25 Aug 2008

Page last updated 4 Oct 2008