Spectrum measurements with DTM firmware

Question

Hi all, we are performing some transmitter spectrum measurements using your nRFGo (PCA10005) board and an Aaronia HF-60105 spectrum analyzer. The SMA connector of nRFGo has been connected to the spectrum analyzer through a 30 dB attenuator. On the PCA10005 board we have uploaded a custom DTM firmware that performs only the TRANSMITTER_TEST.

This is the code of the main file:

#include <stdint.h>
#include <stdbool.h>
#include "nrf51.h"
#include "nrf51_bitfields.h"
#include "ble_dtm.h"
#include "boards.h"
#include "nrf_gpio.h"

#define UICR		0x10001080

int main(void)
{
    uint32_t    dtm_error_code;
    uint32_t	dtm_tx_power = ((*(uint32_t *)UICR) & 0xFFFF0000) >> 16;
    uint32_t	dtm_freq     = ((*(uint32_t *)UICR) & 0x0000FFFF);

    // dtm init
    dtm_error_code = dtm_init();
    if (dtm_error_code != DTM_SUCCESS)
    {
        // If DTM cannot be correctly initialized, then we just return.
        return -1;
    }

    if (dtm_set_txpower(dtm_tx_power) != true)
    {
        // Error
        return -1;
    }

    // dtm transmitter test
    dtm_error_code = dtm_cmd(LE_TRANSMITTER_TEST, dtm_freq, 32, DTM_PKT_PRBS9);
    if (dtm_error_code != DTM_SUCCESS)
    {
        // If DTM cannot be correctly initialized, then we just return.
        return -1;
    }

    for (;;)
    {
        // Will return every timeout, 625 us.
        dtm_wait();
    }
}

Using this firmware the physical channel and the tx power are read from the UICR register (that we write during flashing procedure).

We have found some strange spurs at about TX frequency + 430 MHz (see attached sweep image). This seems not to be the local oscillator which is about 330 Mhz above RX frequency. The spur level is about -23 dBm, much higher than the expected LO leakage.

Can someone explain this strange behavior?

image description

Asbjørn · Accepted Answer

During TX only there shouldn't be any spur there from the nRF51. The LO is only visible in receive mode. The spur is a little high in frequency as well for receive mode on channel 2.403 MHz. The LO frequency is (8/7)* (2400 + channel_numer + 2). So if you are listening on channel 3 the equation would be: (8/7) * (2400 + 3 +2) ~= 2.748 MHz. 
 Could you try reducing the span and "zooming" in on the spur itself. That might give you a more accurate reading for where the spur actually is located and also the level of the spur.

Spectrum measurements with DTM firmware

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