RFX400 daughterboard modification - Revision history

Stan: /* Output spectrum */

2009-12-30T11:41:28Z

Output spectrum

Stan: /* Output spectrum */

2009-12-30T11:39:29Z

Output spectrum

Stan: /* Inductors values */

2009-12-28T22:15:02Z

Inductors values

Stan: /* Inductors values */

2009-12-28T21:53:08Z

Inductors values

Coinchon at 20:42, 28 December 2009

2009-12-28T20:42:42Z

Coinchon: /* Changing the VCO range */

2009-12-28T19:51:28Z

Changing the VCO range

Coinchon: /* Changing the VCO range */

2009-12-28T19:51:10Z

Changing the VCO range

Coinchon: /* Introduction */

2009-12-28T19:13:04Z

Introduction

Coinchon: /* Ouput spectrum */

2009-12-28T18:46:39Z

Ouput spectrum

Coinchon: /* Ouput spectrum */

2009-12-28T18:45:41Z

Ouput spectrum

@@ Line 88: / Line 88: @@
 As different elements of this spectrum are not so easy to filter (very close in frequency) it might be wise to change the center frequency leaving the DAC's (limited by the sampling rate) to make a wider space and use a higher LO frequency.  This to put a fixed mask filter after the DAC's, then low pass filters after the quadrature modulator to kill the LO, the upper beat signal (if we cannot balance perfectly the input source at the modulator) and higher harmonics.  Then we will replace the final stage of the RFX400 board by a monolytic amplifier having a lower 'start frequency'.
 Update:  For the RFX400,  the command to change the offset between the LO from the PLL and the DAC's signal at the input of the quadrature modulator is:  device.set_lo_offset(freq in Hz)
   For example to get an offset of 44 Mhz:   device.set_lo_offset(44000000)

← Older revision		Revision as of 11:39, 30 December 2009
Line 86:		Line 86:
	Possible improvments:		Possible improvments:

−	As different elements of this spectrum are not so easy to filter (very close in frequency) it might be wise to change the center frequency leaving the DAC's (limited by the sampling rate) to make a wider space and use a higher LO frequency. This to put a fixed mask filter after the DAC's, then low pass filters after the quadrature modulator to kill the LO, the upper beat signal (if we cannot balance perfectly the input source at the modulator) and higher harmonics. Then we will replace the final stage of the RFX400 board by a monolytic amplifier having a lower 'start frequency'. ~~TBC.~~..	+	As different elements of this spectrum are not so easy to filter (very close in frequency) it might be wise to change the center frequency leaving the DAC's (limited by the sampling rate) to make a wider space and use a higher LO frequency. This to put a fixed mask filter after the DAC's, then low pass filters after the quadrature modulator to kill the LO, the upper beat signal (if we cannot balance perfectly the input source at the modulator) and higher harmonics. Then we will replace the final stage of the RFX400 board by a monolytic amplifier having a lower 'start frequency'.
		+
		+	Update: For the RFX400, the command to change the offset between the LO from the PLL and the DAC's signal at the input of the quadrature modulator is: device.set_lo_offset(freq in Hz)
		+
		+	For example to get an offset of 44 Mhz: device.set_lo_offset(44000000)

@@ Line 26: / Line 26: @@
 |201'472'364
 |263'463'861
-|not yet done
+|200'300'000
-|not yet done
+|266'000'000
 |-
 |22nH

@@ Line 10: / Line 10: @@
 ===Inductors values===
-Here's a table showing the range of operation depending on the inductance value (derived from ADF4360 formula):
+Here's a table showing the range of operation (Hz) depending on the inductance value (derived from ADF4360 formula):
 {| border="1"
@@ Line 18: / Line 18: @@
 !Min ouput frequency
 !Max output frequency
 |-
 |18nH
@@ Line 24: / Line 26: @@
 |201'472'364
 |263'463'861
 |-
 |22nH
@@ Line 30: / Line 34: @@
 |183'032'682
 |239'350'430
 |-
 |27nH
@@ Line 36: / Line 42: @@
 |165'822'815
 |216'845'219
 |}

@@ Line 56: / Line 56: @@
 Whithout changing the amplifier (remember, we are bellow 300 Mhz the lower limit of the last stage) we still have an average of 5 mW at the output.
-==Ouput spectrum==
+==Output spectrum==
 Here's the output spectrum of the RFX400 after VCO modification:

@@ Line 7: / Line 7: @@
 ==Changing the VCO range==
-If you try to use this card under 400 Mhz you need at least to change the inductances value of the PLL VCO as indicated in gnuradio wiki ([http://gnuradio.org/trac/wiki/UsrpDBoardRFX400 RFX400 modification]. It won't lock outside the original band otherwise. According to the chip data-sheet the correct value to operate on the VHF band III  is around 22 nH.
+If you try to use this card under 400 Mhz you need at least to change the inductances value of the PLL VCO as indicated in gnuradio wiki ([http://gnuradio.org/trac/wiki/UsrpDBoardRFX400 RFX400 modification]). It won't lock outside the original band otherwise. According to the chip data-sheet the correct value to operate on the VHF band III  is around 22 nH.
 ===Inductors values===

@@ Line 1: / Line 1: @@
 ==Introduction==
-The RFX400 can work from 400Mhz to 500 Mhz with a total power up to 200 mW. To use it for the band III on the channels 10,11,12 we need to make some modification.
+The RFX400 can work from 400Mhz to 500 Mhz with a total power up to 200 mW. To use it for the band III on the channels 10,11,12 (see [[Band 3 Channels]]) we need to make some modification.
 The conversion is done with a quadrature modulator (AD8345) taking the two DAC's USRP TX port signal and a carrier from a built in programmable local oscillator (PLL using one ADF4360). The card include also for the TX path 2 stages of amplification before a low pass filter.  The bandwidth of the last stage will not go under 300 Mhz (According to the spec. of the RF3315 rated for 0.3 to 3 Ghz).

← Older revision		Revision as of 18:46, 28 December 2009
Line 73:		Line 73:

	As the signal center frequency (modulated group of carriers from the OFDM signal) coming from the DAC's is quite low (4,7 Mhz) it stays too close to the LO making it hard to filter efficiently. Strong filtering would also probably affect the linearity of the OFDM signal.		As the signal center frequency (modulated group of carriers from the OFDM signal) coming from the DAC's is quite low (4,7 Mhz) it stays too close to the LO making it hard to filter efficiently. Strong filtering would also probably affect the linearity of the OFDM signal.
		+
	We also see that the upper beat signal (LO + 4,7 Mhz) is visible at 30 dB under the lower beat (LO – 4,7 Mhz = wanted signal). This comes from a balancing problem between the two inputs of the quadrature modulator (DC offset ? Averaging in the FPGA when computing the complex signal ? Resistors precision after the DAC's ?) This need to be investigated ...		We also see that the upper beat signal (LO + 4,7 Mhz) is visible at 30 dB under the lower beat (LO – 4,7 Mhz = wanted signal). This comes from a balancing problem between the two inputs of the quadrature modulator (DC offset ? Averaging in the FPGA when computing the complex signal ? Resistors precision after the DAC's ?) This need to be investigated ...

@@ Line 62: / Line 62: @@
 [[Image:dab_sr_32.jpg]]
-We see that working below the original board band will require to change the value of the on-board lowpass filter in order to kill the upper harmonics (that are now under the cutoff frequency of the filter, around 500MHz). The signal with the blue square is the wanted signal while the signal with the red marker is his first harmonic (in the original RFX400 band).The ratio beetween the in-band signal and the first harmonic is also degraded by the fact that we didn't change  the final amplifier stage, further improvments needs to be done...
+We see that working below the original board band will require to change the value of the on-board lowpass filter in order to kill the upper harmonics (that are now under the cutoff frequency of the filter, around 500MHz). The signal with the blue square is the wanted signal while the signal with the red marker is his first harmonic (in the original RFX400 band). The ratio between the in-band signal and the first harmonic is also degraded by the fact that we didn't change the final amplifier stage (RF2250 from RFMD, designed to operate from 300MHz to 3GHz), further improvements needs to be done...
-A closer look to the inband signal:
+Let's now have a closer look at the inband signal:
 [[Image:dab_sr_31.jpg]]
-The LO leakage (center carrier on the picture) remain to high and require a strong attenuation before putting on the air this signal.
+The LO (local oscillator) leakage (center carrier on the picture) remain too high and require a strong attenuation before putting on the air this signal.
-As the signal center frequency (modulated groupe of carriers from the OFDM signal) coming from the DAC's is quite low (4,7 Mhz) it keep the beat too close of the LO, making this hard to filter it efficiently without disturbing the linearity on the OFDM channel.
-We also see that the upper beat signal (LO + 4,7 Mhz) is visible at 30 dB under the lower beat (LO – 4,7 Mhz = wanted signal).  We should have a balance problem between the two inputs of the quadrature modulator (DC offset ? Averaging in the FPGA when computing the complex signal ? Resistors precision after the DAC's ?)  This need to be investigated ...
+As the signal center frequency (modulated group of carriers from the OFDM signal) coming from the DAC's is quite low (4,7 Mhz) it stays too close to the LO making it hard to filter efficiently. Strong filtering would also probably affect the linearity of the OFDM signal.
 As different elements of this spectrum are not so easy to filter (very close in frequency) it might be wise to change the center frequency leaving the DAC's (limited by the sampling rate) to make a wider space and use a higher LO frequency.  This to put a fixed mask filter after the DAC's, then low pass filters after the quadrature modulator to kill the LO, the upper beat signal (if we cannot balance perfectly the input source at the modulator) and higher harmonics.  Then we will replace the final stage of the RFX400 board by a monolytic amplifier having a lower 'start frequency'.    TBC...