Using the best local oscillator - Page 1
Forum section: electronics / rf-microwave

author: arildj78
author_rank: Regular Contributor
timestamp: July 08, 2019, 11:15:56 pm
content: I'm building my first receiver and need some help.The signal is a constant wave at 457kHz ±80Hz. I'm planning to run it through a SA605 or SA606 from NXP. If I use a local oscillator of 912kHz the signal will be mixed down to an IF of 455 kHz where I can run it through a ceramic filter with a narrow bandwidth.Now how should I generate my LO? I haven't found any 912kHz crystals yet.The Gilbert cell in the mixer, does it need a square wave or a sine? If it needs a square, then maybe I can generate a higher freq and then divide it.I have also thought of skipping the mixer and just pass the RF straight to IF filtering. Problem here is that a narrowband filter will attenuate some of the signal itself.Comments are appreciated.

author: radiolistener
author_rank: Super Contributor
timestamp: July 08, 2019, 11:34:53 pm
content: it depends on how clean LO you're needs. In most case it can be done with si5351 synthesizer connected to a cheap controller. For example, for your experiments, you can use kit from these two modules connected together:-https://www.aliexpress.com/item/32972854971.html-https://www.aliexpress.com/item/32656048071.htmlThis cheap si5351 module has not so precise oscillator (25 MHz ±2 kHz) on the board.If you're need more precise, you can buy this one:-https://www.aliexpress.com/item/32817431908.htmlIt can produce any desired frequency with 1 Hz resolution from 5-10 kHz up to 200 MHz and even slightly more.It has 3 channels, so you can produce up to 3 different frequencies simultaneously, but there is some limitation for third channel, because this chip has just two PLL, so it needs to share one PLL between two channels in three channels mode (you can use separate dividers for each channel, but two channels will be driven by single PLL).In order to flash microcontroller, you can use this programmer:-https://www.aliexpress.com/item/32792513237.htmlIf you're not familiar with microcontrollers, let me know I will help.

author: arildj78
author_rank: Regular Contributor
timestamp: July 09, 2019, 03:43:25 am
content: Which waveform is best to feed into the Gilbert cell in the SA605? Sine or square?

author: rf-messkopf
author_rank: Regular Contributor
timestamp: July 09, 2019, 09:01:40 am
content: Quote from: arildj78 on July 09, 2019, 03:43:25 amThe SA605 has an internal oscillator which feeds the Gilbert cell, most probably with a square wave (i.e., does hard switching of the cell). If you connect an external LO at pin 4, the required level is 300 mV according to the datasheet. The waveform will be uncritical as the Gilbert cell is fed through the internal oscillator.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 10, 2019, 04:33:10 am
content: Quote from: arildj78 on July 08, 2019, 11:15:56 pmWhere do I start?I think, with a recap of why we use Superheterodyne techniques:-Early multi tube radios used a technique known as "Tuned Radio Frequency"(TRF).To do this, they used cascaded RF stages to obtain both the selectivity & sensitivity required for continously tunable receiversThere were three main drawbacks to this :-(1) Triode tubes with resonant LC networks in both the grid & anode circuits justwantto oscillate, especially when the tuning of these networks is variable across a wide range of frequencies.This was initially overcome by neutralisation, & later by "screen grid tubes".(2) Cascading tunable RF stages introduces the problem of "tracking".TRFs commonly used three ganged variable capacitor sections, which increased the problem.(3)Both the selectivity & sensitivity of a TRF varies across the tuning range, which is undesirable.Superhets solved some of these problems, minimised others, but one was actually made a bit harder.(1)&(3) By having most of the gain at one (usually lower) frequency, neutralisation was made easier ( only really important pre the advent of screen grid tube).For the same reason, selectivity & sensitivity was made substantially the same across the tuning range.(2) The tracking problem now became one of tuning a local oscillator to be exactly 455kHz higher ( in most cases) than the RF tuning.This was solved by the ingenious use of "trimmer" & "padder" capacitors.Superhets, however, introduced other problems of their own.(a) It was extremely difficult to include the IF frequency within the tuning range of the receiver, due to the production of audible heterodynes ( squealing, etc) as that frequency was approached, so it was pretty much universally avoided.(b) "Image interference"--- a few moment's reflection makes it clear that there are two received frequencies which will produce the required IF at any tuning setting, so that if there are two stations at those frequencies they will mutually interfere.The other side of this is what is known as "double spotting", where the same station appears at two different tuning settings.In MF receivers, the aerial tuning stage before the mixer is usually sufficient to avoid these problems, but early HF receivers using low IFs, usually needed an additional stage of Tuned RF amplification before the mixer.This was obviated in later designs by "double conversion" , where the first IF was quite high in frequency to avoid image intereference, & the second low in frequency for best gain & selectivity.In your case, you are only using fixed tuning to receive 457kHz.Using a 455kHz IF brings both the problems described above into play.(a) Your received frequency is only 2kHz away from your IF.Maybe, just maybe, the carrier suppression of the balanced mixer is enough to prevent any of these adverse effects, but I wouldn't bet the house on it.(b) With a 912kHz LO, the image frequency is 1367kHz--- right in the MF AM Broadcast Band.Maybe you don't have AM Broadcasting anymore, but anywhere that does, you will definitely need a decent bit of selectivity before the mixer.I somewhat doubt that 455kHz filters will be very much down on level at 457kHz, as to the best of my knowledge, they are intended for quite wideband use, from MF AM receivers, to, if the SA605 info is correct,"narrow band" FM.I believe the 455kHz filters are close enough that you could use them in a TRF.I don't know what you are using this for, but the frequency suggests to receive an "avalanche beacon".I wonder what modulation method they use, as, remember, the sA605/606 are designed for FM, with limiting in the IF & FM detection.This paper seems to be trying to do something similar, but they ended up going with an SDR.http://www.diva-portal.se/smash/get/diva2:1288305/FULLTEXT02.pdf

author: radiolistener
author_rank: Super Contributor
timestamp: July 10, 2019, 04:58:08 am
content: Quote from: vk6zgo on July 10, 2019, 04:33:10 amthanks, interesting article

author: duak
author_rank: Super Contributor
timestamp: July 12, 2019, 06:42:28 pm
content: If you use a square wave for an LO, the mixer will also be sensitive to the odd harmonics of the LO in the input signal. Do you have an input filter to attenuate them to acceptable levels?Also, since your IF is so close to the signal of interest, the LO should have low levels of even harmonics.Have you considered using direct conversion?