tuneable LO and fixed IF

In general, the 1st LO should be tuneable in order to obtain the fixed IF. However, the LO can not be continuously tuneable and it has frequency step (120KHz, 20MHz, etc). It seems that it is impossible to get the fixed IF by using the discretely tuneable LO.

How should I understand the case where the 1st LO which has frequency step is used to obtain the fixed IF?

In any case the frequency step must be smaller than IF bandwidth. It must be able to tune exactly to center of received channel.

Hi Borber, I have another question. If RF bandwidth is divided into n singal channel, that is to say, BW(RF) = n * BW(IF) (n = 1,2,3...), then

(1) when the frequency step of LO is just equal to the bandwidth of signal channel BW(IF), or the bandwidth of signal channel is n times than the frequency step of LO, BW(IF) = n * FS, n = 1,2,3,..., the output bandwidth of mixer is the same as BW(IF) which should be needed.

(2) when the frequency step of LO is smaller than the bandwidth of the signal channel and the bandwidth of signal channel is not just n times than the frequency step of LO, what will happen? The output bandwidth of mixer is not the same as BW(IF) which should be needed?

Generally, when channel spacing is constant then LO step can be equal to that spacing.
Receiver bandwidth is determined by IF passband. Mixer output is wider than IF filter and can be determined with input filter or can be wideband without this filter.
Depending on type of modulation a fine tuning is sometimes needed. So the LO step must be smal enough.
Frequency bands I, II, III, IV and V usually do not need fine tuning but Band III receivers often have it.

Therefore,

(1)When BW(RF) = n1 * BW(IF), BW(IF)=n2 * FS(LO step), n1, n2 = 1,2,3,4..., the mixer ouput is just the fixed IF with the fixed BW(IF). And the IF filter is used to choose the signal channel. This case is the simplest!

(2) when FS(LO step)=1/n3 * BW(IF), n3 =2,3,4,5..., how is the LO tuned? The LO is tuned one step after the other step, or the LO should be tuned to the step around the f1(RF)+n*BW(IF)? In this case, How is the fixed IF determined. It is diffcult to choose the right signal channel by IF filter.

(3) what do the Frequency bands I, II, III, IV and V mean?

Whatever you do is right when you are able to tune on channel.
These frequency bands are broadcast bands in Europe. I, II and III are VHF and IV and V are UHF frequency bands for TV and FM radio broadcast.
Practically all frequencies from 160kHz up to 890MHz are divided in bands and are intended for broadcast and comunnications. Channelization is standardized, channels are specified according to specific standard (Europe one others other). Channel width is also specified.

For communications, broadcast and TV, RF bands are divided in many bands which is called channelization and channelization is standardized. channels are specified according to specific standard and channel width is also specified.

So the LO should be tuned in order that the bandwidth of the mixer output is just the bandwidth of channel and IF filter after mixer is for channel choice. That is to say, the LO step is also specified. And LO should be designed with consideration on the specified channel width.

That is the case for communications, broadcast, and TV. How about Radar where there is no channel? How is the LO step determined?

There are lots of ways to do this. It all depends on what you are receiving. If we are talking about broadcast band reception, as stated above, the channels are all spaced at specific channel centers in a particular country, and you just have to have the right step size to get them all.

If you are talking about some sort of broadband receiver that might need a small step size you can:1) use a fractional N synthesizer or a DDS based synthesizer to achieve a 1 Hz step size, or 2) have a course step size, a bigger IF bandwidth, and do the final filtering and downconversion digitally in a DSP chip.

If you are talking about a radar system, since you are generating the transmit signal, you can sample off some of the transmit frequency and use it to become the recievers local oscillator--achieving phase and frequency conherence between transmit and recieve signals.