[Hackrf-dev] interested in lower minimum frequency (LF range)

[Paul Connolly]

A spyverter would work (maximum power it could handle would be 200mW or
+23dBm and the maximum that the HackRF One can TX would be 15dBm -
https://github.com/mossmann/hackrf/wiki/HackRF-One#transmit-power ). A
spyverter would normally map DC-60MHz to 120MHz-180MHz but it can work
in reverse, just like a ham-it-up, 120MHz-180MHz to DC-60MHz. LF being
30kHz to 300kHz is a bit low for the ham-it-up, it was designed to
convert signals in the MF and HF Bands (0.5MHz to 50MHz) to signals in
the VHF Band (125.5MHz to 175MHz). The ham-it-up will work, but with
more attenuation in the band that interests you (
https://code.google.com/p/opendous/wiki/Upconverter ).

You would still need a bandpass filter on the output before feeding it
into an antenna to avoid spewing the DC spike (and other birdies) into
the airwaves. So you would need to offset tune your signal away from the
central DC spike, making sure that the DC spike is well inside the
attenuation of the BPF.

Lets say that you wanted to broadcast a AM signal at 100kHz with 9 kHz
bandwidth.
(ignoring any ppm offsets of the spyverter[or ham-it-up] and the hackrf
clocks for now, it is something that you will need to deal with)
So the signal that you want to TX would be at 120.100MHz(or 125.100MHz
on a ham-it-up) on the HackRF One.
And if the bandwidth was 8MHz on the HackRF and the centre frequency
could be tuned to say 122.100MHz so that the DC spike would be 2MHz away
from the signal that you want to TX.

You sould  make a BPF circuit that sits between the spyverter(or
ham-it-up) and the antenna - see below:
You can make a reasonable basic bandpass circuit with 3 inductors and 3
capacitors.
(Random site found from a search for "bandpass filter calculator")
http://www.changpuak.ch/electronics/butterworth_bandpass.php
I gave it the following parameters:
Center Freq. [MHz] 0.1 (100kHz)
Bandwidth [MHz] 0.009   (9kHz)
Impedance [Ω] 50
Order [1-19] 3

And it returned the following output:
--- start of result ---
Butterworth Bandpass Filter
www.changpuak.ch/electronics/butterworth_bandpass.php
Version : 10. Jan 2014
----------------------------------------------------
Center Frequency : 0.1 MHz
System Impedance : 50 Ohm
Order of Filter  : 3
----------------------------------------------------
Element 1 , Orientation : shunt
C = 353677.651 pF, L = 7161.972 nH
Element 2 , Orientation : series
C = 1432.394 pF, L = 1768388.257 nH
Element 3 , Orientation : shunt
C = 353677.652 pF, L = 7161.972 nH
----------------------------------------------------
Appendix : Prototype G values
G[1] : 1
G[2] : 2
G[3] : 1
--- end of result ---

Approximate real component values can be substituted for the above, and
it should still work well enough, but you should really test the circuit
in a spice simulator to be sure of its frequency response.
Element 1 ,(360nF and 75uH)    
Element 2 ,(1.5nF and 1.8mH)
Element 3 ,(360nF and 75uH)

.11.2.2.33.
...........
>++-L-C-++>
.||.....||.
.LC.....LC.
.||.....||.
.GG.....GG.
...........
.11.2.2.33.

But if you are going to TX you should have a amateur radio license, in
most countries you would learn everything that you need to know in the
process of getting your license.

On 22/11/2015 11:16, Dominic Spill wrote:
> On 18 November 2015 at 13:40, Frank Ch. Eigler <fche at elastic.org> wrote:
>> I'm interested in receiving and low-power transmitting with the HackRF
>> at LF-range frequencies below, the 1MHz floor.  What are the limiting
>> factors in the hardware or firmware that would impede transmitting a
>> 100kHz AM signal?
> The HackRF One transmit path passes a stream of complex bytes to two
> DACs, then combines these two analogue signals and finally mixes the
> signal up to the desired transmit frequency (in fact, HackRF shifts
> the signal twice, but that's not important for now).  The hardware
> limitations will be filters at the baseband and at the mixing stage,
> which will filter out your LF signal.
>
> For this sort of operation I would normally recommend a ham-it-up
> device[1], but even they only go down to HF/MF.  Perhaps other
> upconverters are available, but I haven't seen any to recommend.
>
>> As I understand it, receiving is probably OK via decimation in software.
> Have you tried this with HackRF One?  I would assume that you will
> find the same issue with filters.
>
> Dominic
>
> [1] https://www.nooelec.com/store/ham-it-up.html
> _______________________________________________
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> HackRF-dev at greatscottgadgets.com
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