{"id":6739,"date":"2020-04-12T17:00:29","date_gmt":"2020-04-12T11:30:29","guid":{"rendered":"https:\/\/nuclearrambo.com\/wordpress\/?p=6739"},"modified":"2024-10-03T15:18:27","modified_gmt":"2024-10-03T09:48:27","slug":"a-simple-hf-up-converter-with-limesdr","status":"publish","type":"post","link":"https:\/\/nuclearrambo.com\/wordpress\/a-simple-hf-up-converter-with-limesdr\/","title":{"rendered":"A simple HF up converter with LimeSDR"},"content":{"rendered":"\n

In times like these when we are all stuck in our homes, most of us are becoming active on the radio. The best ideas come out when we are challenged and right now we are all challenged mentally, thinking what to do during the indefinite lock down. Anyway, today I will demonstrate how to build a simple up-converter with LimeSDR Mini. This same method will work if you have any other transmit capable SDR. <\/p>\n\n\n\n

The basic frequency converter<\/h2>\n\n\n\n

The commercially available HF up-converters have a nice little filter in them that filters out FM band. Nonetheless, you can still do without a filter at a risk of having FM signals contaminate the spectrum a bit. That can be avoided if you choose your LO by trying different frequencies.<\/p>\n\n\n\n

\"limesdr<\/a>
A basic up-converter<\/figcaption><\/figure>\n\n\n\n

The mixer multiplies the RF signal with the LO and produces sum and difference products. In my case the frequency range of interest lies between \\(7MHz\\) and \\(21MHz\\). As a result, the mixer will produce frequency components around the LO in the following manner. Note, that the LO frequency is set at \\(450MHz\\).<\/p>\n\n\n\\(Sum\\,frequency\\,range = 450MHz + 7MHz\\,and\\, 450MHz+\u00a0 21MHz\\)\n\n\n\\(Difference\\,frequency\\,range = 450MHz – 7MHz\\,and\\, 450MHz-\u00a0 21MHz\\)\n\n\n\n

In other words, the sum frequencies are between \\(457MHz\\,and\\,471MHz\\) and vice versa for the difference frequencies.<\/p>\n\n\n\n

The LO signal needs to be sufficiently strong in case of a passive mixer that I plan on using. LimeSDR produces about 10dBm transmit signal sufficient to drive the mixer.<\/p>\n\n\n\n

There is another point to note in the block diagram, that is the LNA. The LNA stands for Low noise amplifier. Why do we need an LNA? To answer\u00c2  that, we need to look at the Frii’s formula<\/a>. It states that the noise figure of the first stage of the receiver significantly affects the noise figure of the entire system. In simpler words, if the noise figure of first stage is low, the over all noise figure of the receiver system also remains low. Since, the noise figure of a mixer is very high, it can reduce the sensitivity of our receiver.<\/p>\n\n\n\\(NF_{total} = NF_1 + \\frac{NF_2 -1}{G_1}+\\frac{NF_3-1}{G_1\\times{G_2}}…\\)\n\n\n\n

The real calculations<\/h2>\n\n\n\n

We went through some theory up there. Now, it’s time to apply that on practical components. I will be using the Minicircuits ZFL-500HLN<\/a> in the first stage that has a gain of \\(20dB\\). Although, the datasheet states the start frequency as \\(10MHz\\), the S-parameter file tells me that the amplifier is good down to \\(1MHz\\). The amplifier offers a noise figure of \\(3.8dB\\) which isn’t as low as it should be. Nonetheless, it’s better than feeding the signal directly to a mixer.<\/p>\n\n\n\n

For the mixer, I have Minicircuits TUF-3LH<\/a> having a conversion loss of approximately \\(5dB\\). In case of mixers, the noise figure is roughly equivalent to the conversion loss. Let us, put this information into the Frii’s formula and see what \\(NF_{total}\\) we get. The noise figure values need to be converted into linear scale before we put them into the Frii’s formula.<\/p>\n\n\n\\(NF_{linear} = 10^{\\frac{NF_{dB}}{10}}\\)\n\n\n\\(NF_{total} = 2.3988+\\frac{3.16-1}{100} = 2.42\\)\n\n\n\n

Converting it into dB scale, \\(NF_{total} = 10log_{10}(2.42) = 3.83dB\\)<\/p>\n\n\n\n

Had we fed our signal straight into the mixer, our noise figure would be >5dB.<\/p>\n\n\n\n

NOTE:<\/strong> People are going to complain that I did not take into consideration the NF of the receiving port of the SDR. Yes I didn’t for two reasons. First, it is not going to significantly affect the number. Second, I couldn’t find the required data.<\/p>\n\n\n\n