WHAT IS A DIPLEXER?

In my research into diode DBM's {Double Balanced Mixer} (and CMOS mixers)
the following advice is given:
The most critical port is the I.F. port which MUST see 50 ohms at all frequencies.
The simple solution here is to place a DIPLEXER at this port together with a post mixer amplifier.
The WIKIPEDIA explanation of a DIPLEXER is:
A passive device that implements frequency domain multiplexing.
Two ports (e.g., L and H) are multiplexed onto a third port (e.g., S).
The signals on ports L and H occupy dis-joint frequency bands.
Consequently, the signals on L and H can coexist on port S without interfering with each other.
Typically, the signal on port L will occupy a single low frequency band
and the signal on port H will occupy a higher frequency band.
In that situation, the diplexer consists of a lowpass filter connecting ports L and S
and high pass filter connecting ports H and S.
Ideally, all the signal power on port L is transferred to the S port and vice versa.
All the signal power on port H is transferred to port S and vice versa.
Ideally, the separation of the signals is complete.
None of the low band signal is transferred from the S port to the H port.
In the real world, some power will be lost, and some signal power will leak to the wrong port.
Television diplexer consisting of a high-pass filter (left) and a low-pass filter (right).
The antenna is connected to the screw terminals to the left of center.

The diplexer, being made with passive devices, means ports can be either as inputs or outputs.

Consider the 2 following circuits, Ref 1 & Ref 2 (see below)
which are CMOS frequency mixers followed by a DIPLEXER .
click for full circuit
The Diplexer seems to be an AC coupled, 50 ohm resistive load
(satisfies the requirement "I.F. port which MUST see 50 ohms")
which feeds an LC parallel circuit resonant at the I.F. design frequency.
Therefore the unwanted (sum or difference) mixer product (and any other spurious signals) would flow to ground, and because of the high Impedance of the LC tuned circuit, the IF signal would pass on into the I.F. amp
(satisfies the requirement "a post mixer amplifier")

Ref 1: 160m superhet receiver - Amateur Radio (Australia) Mag' May 2002
Ref 2: HF receiving convertor - Amateur Radio (Australia) Mag' June 2002
Both Drew Diamond/VK3XU designs

to Diplexer Pg 2
to Diplexer Pg 3
to Diplexer Pg 4
to Diplexer Pg 5
to Diplexer Pg 6