Any issue about this mixer design?

There is a mixer design, as the picture.
The working freq is 200M~300M，"RF REF" is LO, LNA1 is RF, then merge IF+ and IF-.
1. Is any DC level at IF+/-?
2. If I put a LPF after the mixer, do I still need a AC-couple capcitor to isolate DC?
3. I hope to get diff signals on IF+ and IF-, is the circuit OK for that purpose?
Many thanks.



4. How much power need for LO to drive the mixer? If the power is very low, does the mixer still work?

I have never seen a successful diode switching mixer circuit operating at those kind of frequencies that did not use very carefully made input and output transformers, very carefully impedance matched at all three ports.

As you can readily buy these diode mixers with guaranteed performance, I think you are going about all this the hard way.

You can certainly have dc on the output, if the two input frequencies are the same, or some dc shift if both inputs are exactly harmonically related.
Basically its a beat frequency which then becomes effectively zero, and then the whole thing behaves a bit like a synchronous rectifier.

Your oscillator input needs to be very high, enough to drive all four diodes hard into and out of conduction. several volts of oscillator power will be required to do that.

A switching mixer will certainly produce IF+ and IF- and also both input frequencies plus harmonics and intermodulation products.
You get the whole lot.
Its only by very careful circuit balance, and careful control of impedances and stray reactances that the unwanted rubbish can be minimised.
That is going to be a very big problem for you, especially as you have not used the more usual transformers.

If all you want is IF+ and IF- a much better way would be using a multiplier type of mixer, not a switching mixer.

Hi，Tony,
would you suggest one multiplier mixer? Part number of which vendor?
Thanks.



Obviuosly the original mixer is very cheap,
could you comment some improvement on the original design?
Or could you recommend another very cheaper mixer? which can work at 300MHz?
Thanks.

There are plenty of choices, but how about an MC1496 made by Motorola, ON Semiconductor, and others.
These mixers are rated to 300 Mhz and offer excellent carrier suppression without adjustment, and no transformers are required.

Another very popular mixer (with quite a lot of gain) that may be a lot easier to use would be NE602A or SA602A.
Both these are linear analog multiplier types that require far less oscillator power, and don't create the multiple harmonics in the output that switching mixers all produce.

Almost anything will "work" but probably not very well.
It depends what you want to use it for.
If its for audio frequencies its all a lot easier, but at 300 Mhz, and unless you have a narrow band filter right after the mixer, all the rubbish coming out of a cheap inefficient mixer may cause some problems.

Cost is always an issue, but being located in China, you must have access to the lowest cost parts anywhere in the world.

HI，
I need a low power consumption, a low LO drive, double-banlanced Mixer.
Working freq is about 300MHz. IF is about hundreds Hz.
And the size should be small.
Who can recommend one such mixer?
Many thanks.

Please look at "www.minicircuits.com". They have many excellent mixers with various LO drive levels and good specifications. I have used several of their model to a full satisfaction.

Hi, Jiripolivka, Thanks.
I found Skyworks mixers “SKY73049_350LF_201130F Mixer”.
In the datasheet, there is the following description:
"The SKY74039 has differential, high impedance open collector
outputs (IF_OUT_N and IF_OUT_P) that need to be pulled up to
supply voltage through RF chokes or through the center tap of a
balun. The IF load for the mixer is provided externally."
1. I want to try this mixer with hundreds Hz IF, although in datsheet IF min freq is 50M, any risk?
2. I can't use inductor as RF chokes between VCC and IF, so I want to use pull-up resistors to do so, what value is OK? 1KR or 10KR? Or must use inductors?
Best,

You can certainly try whatever you want but do not expect good results if out of spec range.
Inductors offer lower conversion loss and are needed to feed the transistors in the mixer.

I think you should follow the specifications and use good filters. If you can get expected results, then try modifications. Otherwise poor results or damage can result.

Thanks.
And another question, I want to update an original design, which is amplify a weak signal with a dual input instrument amplifier, because the original is a banlanced mixer with two IF output.
If I use a non-banlanced mixer, i.e. only one IF output, then I use one input instrument amplifier.
How much I may damage the the weak signal?
Any other points need emphasis when design?

As you never indicated the IF frequency range, remember that instrumentation amplifiers are designed for D C and audio range. A good opamp like NE 534 can be better.
How "weak" your signal is? What is signal bandwidth? It determines the thermal noise. Then mixer conversion loss reduces input signal by around 10 dB. Maybe you need filters and matching circuits.

To design a good receiver you need a good knowledge, see e.g. ARRL Radio amateurs¨¨handbook.

Hi, Jiripolivka,
Thanks for the reply.
I have said that "IF is about hundreds Hz." in the #5 post.
I preferred a mixer which needs +7dBm LO power, but now the LO is only -20dBm. Can the mixer(MAC-24+) work?
I guess no, because the LO powe can't turn on the diode of the mixer.
So how about mixing my LO with another signal (which is 0dBm), let the latter signal turn on the mixer, and so that my signal can mix with RF signal? Is that OK?
Thanks.

I still think you would be better with a multiplier type of mixer.
Differential output, response right down to dc, and some conversion gain.
Also very low oscillator power requirement.

For a hundred Hz you can use the instrumentation amplifier. I prefer the NE 5534 low-noise opamp.
Mixers cannot function with a low LO level. This is why some mixers have LO buffers.
If you combine two or more mixers, the spectrum will be messy. Use one mixer and follow the specs. Use good filters for RF and IF, and a clean strong LO.

ADE-2M+ is a Bi-Phase (0deg and 180 deg) modulator, can I use it to be a down-converter to get the diff IF signals?



Hi, Warpspeed， I can't find a modern multiplier type of mixer, it is usually too big.
And the mixer has a OSC (but I don't need that) or LO buffer, which also consump crrent, the same as use a LO driver amplifier.

Please look on datasheet of ADE-2M+ mixer! It can be used as a standard mixer OR a bi-phase mixer.
There is no need of anything else than giving it +7 dBm of LO power and some filters as needed.

I used similar mixers from this company with excellent results every time.

Hi， Jiripolivka，
Thanks.
I found that Mini-Circuits said ADE-2M+ is a modulator (up-converter), so I want to confirm if it can be a de-modulator.
Sometimes modulator is different from de-modulator, just as some up-converter can't be down-converter.
Thanks again.

Mixers like these can be used as "standard" mixers, or bi-phase mixers as explained in data sheet . Most mixers can be used as up or own converters. For upconverters, the difference is in input power level to be upconverted- then this level should be 7-8 dB lower than LO input level to save linearity.

Mixers due to their non-linear elements (diodes, transistors) generate a linear combination of frequencies from their input signals. Filters must be used to select desired and suppress undesired spectrum components.