Frequency mixing at low frequencies (with AD835)

I think that most of the Zero IF receiver IC's that are available have ADC's build in, so you actually have a digital output. Even without this, ZIF receivers have IQ baseband outputs, where you have 2 analog output's for a single receiver path, which would require a IQ modulator to upconvert this into a Lo-IF approach. It's my impression that "wideband" receivers that cover 150-900 MHz have multiple conversion stages, as well as IF filters, so that they would only work over a subset of that frequency range at one time, perhaps 2 to 10 MHz within that 150-900 MHz.

You might want to look at the Lime LM6002 transceiver part, which would give you a Digital I/Q output, which would possibly be easier to deal with in a signal processing point of view, as you have a single channel of digital information to deal with for each channel.

What is your application? From what you've mentioned so far, I'm going to go out on a limb and say that it sounds like you're trying to build a 14MHz local oscillator for a quadrature mixer/detector in the ham radio 20 meter band, so I'll address some issues specific to that application (though these issues are not exclusive to that application).

First, to answer your question, you'll want to get the 14MHz signal generated first, then apply the 90 degree phase shift. There are a couple of reasons for this, including, but not limited to:

1. Once you have your in phase ("I", aka, not-phase-shifted) and quadrature ("Q", aka, 90 degree shifted) signals, you now have two separate signals, so if you want to take a 500kHz signal and use mixers to multiply it up to 14MHz, you now have to have two mixer chains, one for each signal.
2. Your two mixer chains now have to be identical in every respect in order to ensure that no additional phase changes occur between your I and Q signals. Physics/murphy make this difficult.

So, it's easier and more practical to do the phase shift last. Since it sounds like you just need a fixed-frequency oscillator, this can be done pretty simply with passive components; see figure 6 (and the corresponding text) from this ARRL article.

You could also use a pair of D-type flip flops to generate your I and Q signals, which would work over a wide range of frequencies, but it requires your oscillator to be running at 14*4 = 56MHz; there's an example of this in the "Generating I/Q Quadrature Local Oscillator Signals" section of this page.

Now, even if you did one of the above and got your 14MHz I and Q signals generated, I think you would not be very happy with the result. Although you didn't mention it, from your description I'm guessing that your 500kHz op-amp oscillator is some form of RC relaxation oscillator? Those have terrible frequency stability (with respect to, say, a crystal oscillator), and to make matters worse, multiplying the signal up also multiplies the frequency instability!

Likewise, specially WRT local oscillators, phase noise is very important; multiplication of a signal will increase (worsen) the phase noise by a factor of 20*log(N), where N is the multiplication factor. Without going into too much detail on phase noise, as with most forms of noise, less is generally better.

So, given the fact that it sounds like you need a fixed-frequency oscillator on/around 14MHz, I would strongly recommend building a crystal oscillator. The Pierce oscillator design is particularly popular, as it is hard to beat in terms of simplicity and performance. You can use a jellybean transistor (e.g., 2N3904) for the amplifier, or use an inverting logic gate IC; there is a related question here which may help you get started; you can then do either a passive/active phase shift circuit, as mentioned above, to get your I and Q signals.

BTW, if my hunch about your application is correct, I suspect your comment "I'm afraid it has to be a sinusoid" is probably incorrect. Most mixers actually work better with square wave LO input, as it switches the mixer's diodes/transistors on faster.